Idiomatic dictionary definitions of 'downhill'
Christine Ammer, The Facts on File Dictionary of Clichés, second edition (2006) has this interesting remark about the prevailing sense of downhill in English:
go downhill Deteriorate, decline. Although it would seem that going down a hill is easier than going up, downhill has meant a decline since the 1500s, although Daniel Defoe also used it in the sense of easy ("a very short cut, and all down hill," Robinson Crusoe, 1719). An 1856 history of England had the sense of declining: "The monks had traveled swiftly on the downhill road of human corruption."
John Ayto, Oxford Dictionary of English Idioms, third edition (2009) neatly captures the opposite tendencies of "all downhill" (or in this case, "downhill all the way"):
be downhill all the way (1) be easy in comparison with what came before. (2) become worse or less successful.
go downhill become worse; deteriorate.
The order of Ayto's entries suggests that the positive sense of "downhill all the way" is older than the negative sense, which in turn suggests that the negative sense may have arisen through the influence of "go downhill," which idiomatically in English has only a negative connotation.
Figurative use of 'downhill' in early Google Books search results
The earliest match for any of these expressions in Google Books search results appears to involve "go downhill"—already in its negative sense. From J. Morgan, A Compleat History of the Present Seat of War in Africa, Between the Spaniards and the Algerines (1632):
They [the Spaniards] were glad to content themselves with holding what they had got, and now and then venturing out, accompanied and conducted by their Moorish Allies, to surprise and bring away into Slavery their sleeping Neighbours. Otherwise, they soon began to cut as contemptible a Figure in Barbary, as the Grand Spanish Monarchy has, in Proportion, done in Europe, since it began to go down Hill, which we may date from the memorable 1588, when its Not-invincible Armada made our Great Great-Grandfathers that friendly Visit.
Much later, but to similar effect, is the argument of appellant's lawyer A. Spencer in Whelan v. Whelan (April 1824) in Cases in the Court of Errors of the State of New-York (1825):
The facts bring these parties within the principle which required the whole of this business to have been done with great deliberation, publicly, and under the advice of mutual friends. It is said we do not shew actual imbecility. This is not necessary. Here is a man 75 years of age. We have the authority of the constitution, that at 60 a man begins to go down hill ; and on this ground he is excluded from the superior benches of justice. The respondents should have rescued the appellant from the operation of this general rule, by shewing that he was an exception. Besides, credulity is a conformation of the mind as fatal as general imbecility.
In The Life, and Strange Surprizing Adventures of Robinson Crusoe, of York, Mariner (1721), as Ammer notes, Daniel Defoe uses "down hill" in a sense indicating "easy"; but he is also using the word literally—that is to say, topographically—to describe descending a hillside:
It came now very warmly upon my Thoughts, and indeed irresistibly, that now as my Time to get me a Servant, and perhaps a Companion, or Assistant, and that I was call'd plainly by Providence to save this poor Creature's Life; I immediately gat down the Ladders with all possible expedition, fetch'd my two Guns, for they were both at the Foot of the Ladders, as I observ'd above; and getting up again with the same haste, to the Top of the Hill, I cross'd toward the Sea ; and having a very short cut, and all down Hill, clap'd my self in the Way between the Pursuers and the pursued, hallowing aloud to him that fled, who looking back, was at first perhaps as much frighted at me, as at them; but I beckon'd with my Hand to him to come back, and in the mean time, I slowly advanced towards the two that follow'd; then rushing at once upon the foremost, I knock'd him with the Stock of my Piece; I was loth to fire because I would not have the rest hear; tho' at that Distance it would not have been easily heard; ...
However, downhill is used in the sense of "easy and natural" in Levi Woodbury, "Speech of Mr. Woodbury, of New Hampshire, in the Senate of the United States, February 23, 1830, on Mr. Foote's Resolution" (1830):
If the gentleman on my right, (Mr. BARTON,) seeks by such measures to pull down this administration, he may not find it so "downhill a business" as he represented the pulling down administrations in this country usually to be. Perhaps it would be well, before further taunts of this kind are repeated, to set history right, and to recollect that pulling down administrations in this country has never proved quite so easy and downhill a business as seems to be supposed when the administrations have been democratic—not a very downhill concern when it was attempted on either Mr. Jefferson's, Madison's, or Monroe's administration—but rather easier to be sure—rather more of a downhill concern in the two four-tear administrations in this country, suspected, at least, of no very great devotion to some of the leading principles of democracy. ... And this "downhill business" may prove an uphill job to the undertakers. At least if this administration is ever by such leaders, and in this way rolled to the bottom of the hill, I may as a Yankee, be allowed to guess, that those leaders, like Sisyphus, will find it must speedily be rolled back again.
It sometimes appears that "all downhill" can simply mean "swiftly and with little resistance, whether to a good end or a bad one," as in this instance from Philip Bailey, Festus: A Poem (1857):
FESTUS. Aspiring! You will find/ The world is all uphill when we would do;/ All downhill when we suffer. Nay, it will part/ Like the Red Sea, so that the poor may pass./ We make our compliments to wretchedness,/ And hope the poor want nothing, and are well.
There is at least some possibility, however, that "downhill" may have been long associated with the idea of heading in the direction of perdition, given ancient notions of heaven being in the sky above and hell under the earth below. That is certainly the implication of this excerpt from "Prelude to the Plays: Or. A few Serious Questions proposed to Gentlemen and Ladies, and others, that frequent the Play-House" (1729):
Q. Will this merry life hold always? What will be the End hereof? Comicum Principium, finis Tragicum: This comical Life is like to end in a dreadful Tragedy. Facilis descensus avernio: The Way to Hell is all down Hill. The Bee may be drowned in its own Honey; So may we in immoderate Pleasures of this Life. She that liveth in Pleasure, is dead while she liveth. It is good to be merry and wise. The most solid and lasting Joy arises from Knowledge of the Pardon of our Sins, and our Relation to God in Christ, by Faith, and our Title unto eternal Life. Joy and Peace in believing: This is what the World can neither give or take away.
However, although several other authors cite the expression Facilis descensus avernio, no one but the anonymous author of this tract translates it in his striking "all down Hill" way. On the other hand Archbishop Robert Leighton uses a different Latin passage to reach a similar conclusion in "The way of Sin down Hill," in The Select Works of Archbishop Leighton: Prepared for the Practical Use of Private Christians (1832):
The way of sin is motus in proclivi, down hill: a man cannot stop where he would; and he that will be tampering with dangerous occasions, in confidence of his resolution, shall find himself often carried beyond his purpose. All the other precepts of this Law remaining in full force in their proper sense, it cannot but be an injury done to this
Leighton died in 1684, so this is an even earlier example than the "Prelude to the Plays" instance.
'Downhill all the way' and 'all downhill from here/there'
The earliest of these longer expressions is "downhill all the way," and it comes up fairly regularly in nineteenth century publications referring to roads and rivers that do indeed drop continuously in altitude from one specified point to another. The first figurative use of "downhill all the way" that a Google Books search turns up is from The American Magazine (1943) [combined snippets] and uses the phrase in a negative sense:
They didn't try to argue with him, they went silently, almost hurriedly, and when Clara closed the door upon them she was afraid to look at Roger, afraid and ashamed. She thought, Now he's disgusted with me. And why not? I can see so clearly now how I've wasted my life, how I've let things slide. I've been going downhill all the way. He knows it, too. He knows I'm just a desperate, lonesome female living a shabby excuse for a life.
Notice, however, that in this instance "downhill all the way" isn't preceded by is or are, as in a description of a road or trail, but by going, and we have already seen that "go downhill" has a long history of negative figurative use.
"All downhill from here [or there]" is a somewhat more recent phrase, to judge from publishing database results. The earliest match I could find in an Elephind search is in this meandering reportage from Smith, "Sparks from the News Anvil," in the [Launceston, Tasmania] Examiner (June 12, 1946):
And it is with this thought we leave the Victory Show and go to Queenstown, looking for a motor bike for a policeman in a hurry. This is a bad business. I heard a little girl the other day ask her mother to bring something back on the radio because she had just missed the session. That item was like the one motor-cycle outfit at Queenstown. but it's really at Gormanston, probably because it's all downhill from there. If that motor bike has gone just before a good murder has been committed, it's just too bad. There is not even a hint that it is fitted with a radio in the case of its having to be in two places at once. I hope the Attorney-General will get on with his reviewing.
The usage here seems to be literal (Queenstown is uphill from Gormanton), but the writing is not of the best quality. Better is this instance from Jon Cleary, The Climate of Courage, serialized in the Sydney [New South Wales] Morning Herald (December 16, 1953):
"How do you feel, mate?"
"Crook." All the cocky lightness had gone out of Mick's voice. "I've dropped me bundle, Joe."
"Don't start talking like some weak-kneed sheila," Joe said. "You've come this far. It's all downhill from here. You'll be able to coast all the way."
This is again downhill in the literal sense of descending elevation.
However, the next two matches use the phrase figuratively—and in a negative sense. From "Bomber Nine Insures Winning Season by Upending Hartwick," in the [Ithaca, New York] Ithacan (May 21, 1965):
The victory brought the Bomber's record to 10-6 and boosted the morale of a team that had just completed a rough week on the road. They started it off auspiciously enough by defeating Susquehanna, 11-6 and Cortland 5-4, but it was all downhill from here. They came out on the short end of a 14-13 slugfest at East Stroudsburg, then lost to Villanova 6-2 and to West Chester 8-7. Errors and ineffective pitching were among the reasons for the three game slide of the Ithacans.
And from "Gymnasts Rout Falcons as Cohen Tallies 57.2," in the [University Park, Pennsylvania] Daily Collegian (February 1, 1966):
The Air Force figured to win the team trampoline score by three points or more, but the Lions' Gene Scofield, Marty DeSantis and Dave Coggshall kept the deficit to 1.25. And it was all downhill from there for the flyboys.
Higgins and Barry MacLean alone broke through the clouds of a dismal night for the Falcons by copping the parallel bars and trampoline, respectively.
An arguably positive instance of the phrase appears in "K-State Flops," in the Columbia [Missouri] Missourian (March 18, 1973):
The [Memphis State] Tigers, with Kenon, Bill Buford and Wes Westfall all on the sidelines with four fouls, put the game away midway in the second half when they outscored the [Kansas State] Wildcats 10-0 over a three-minute span.
That made it 66-47 and it was all downhill from there.
In this article, whether "all downhill from there" is a positive or a negative depends on whether you view it from the Memphis State or the Kansas State perspective, but the final phrase "and it was all downhill from there" reads as though it were interchangeable with "and the Tigers coasted [downhill] from there."
Similarly ambiguous (but probably positive) is this instance from "Wildcats Go 2-4 on Road," in the [Abilene, Texas] Optimist (January 11, 1984):
"We feel like it's all downhill from here on," said ACU women's basketball coach Burl McCoy. The Wildcats finished 2-4 after a tough road schedule. The women's next game will be at home against Texas Lutheran at 6 p.m.
Although a reader might infer that the Abilene Christian University coach was throwing in the towel six games into a 25-game season, it is far more likely that he meant that the team had completed the most difficult part of its schedule and that the rest of the season would be easier sledding. The remainder of the article confirms that this is what the coach meant. Nevertheless, in the vast majority of newspaper instances, writers and interviewees use the phrase in a negative sense.
If you are on a hill at high elevation and you want to be farther down the hill or off it entirely, "going downhill" is a good thing, as is a path that runs "downhill all the way" or "all downhill from here." But the fact that going downhill is generally thought of as being easier than going uphill doesn't mean that it is inherently desirable. Figuratively, "going downhill" has long been associated with deterioration or decline, and it has even at times suggested a trajectory similar to "going to Hell."
Examples of negative and positive figurative use of "all downhill from here [or there]" go back at least to the 1960s and 1970s, respectively. The vast majority of instances of figurative usage seem to use the phrase in a negative sense, however.