For an interesting view of how authors and book publishers viewed the practice of adding italics to quotations in the early nineteenth century, see the Preface to James Plumptre's book Letters to John Aikin, M.D. (1811). In it, Plumptre notes the objections of a friend of his (whom he had asked to review the manuscript prior to publication) to Plumptre's practice of adding italics at various places in Dr. Aikin's correspondence to emphasize what Plumptre considered salient points. Plumptre then writes:
Having always been accustomed to add Italics myself, and seen them in the works of others, especially in Reviews, I pleaded precedent to my friend, and said that I thought the matter was so well understood as not to mislead. To this my friend replies,
"I am sorry we do not agree about Italics added by the Quoter. You speak of it as an established mode. I own it is (in my judgment) far too frequent: but, I think, some to whom I have stated my ideas have allowed the practice to be wrong. It is, with me, clearly and decidedly wrong, beyond the power of authority or custom so to establish it as to make it right; though I fancy I have been one of the most obedient men, through life, to things established,—particularly in language:—and Italics (we agree) are a part of language. You say "it appears to me to be sufficiently understood not to mislead." My grand objection is that it does mislead, or may do it. If such a rule on this point be not observed, how should you, or any reader of the few words I have just quoted from your letter, know whether the two words were underlined by you or by me? ... I am not sure that I should not wish your Work suppressed, rather than printed with this blemish, as I call it."
On farther reflection, these remarks appeared to me to be so very just, and to bear with so much force upon works of controversy in particular, that I thought it but justice to the author whose works I criticized to remove the additional underlinings which I had inserted, and to leave those only which were the author's. In doing this, I found great difficulties present themselves, and endeavoured to form some rules by which I might regulate my quotations; but this required more time and consideration than I could give the subject; and all that I have been able to do in the present instance has been to adopt such modes of calling the reader's attention to particular words and passages as the case seemed best to admit; sometimes by repetition, and sometimes by printing particular words in Italics, when I was not using inverted commas, the professed marks of quotation of faithful copying.
From this discussion we may conclude that, as of 1811, authors frequently added italics to quotations without informing readers when they did so, and that no system for explicitly identifying such alterations was in widespread use.
Nevertheless, at least one author had already worked out essentially the form that would later become standard—enclosing the attribution in parentheses immediately after the quotation. From "Sir Herbert Croft's Letter to Mr. Nichols," in The Gentleman's Magazine, volume 70 (1800):
That, four months after, without any apparent reason for a robber's continuing such a correspondence, I "again wrote to justify myself" (my italics); where, by slipping in the word again, an epic poet asserts, without showing any such thing, that I had justified myself, before, from a charge, the hinting of which would certainly have prevented my ever sending the 10l.
To judge from the scanty results in Ngram Viewer "(my italics)" was more common than "(italics mine)" as a standard acknowledgment form until the mid-1880s: Among sources published between 1800 and 1878, ten use "(my italics)" and only two use "(italics mine)"; thereafter, "(my italics)" quickly lost its advantage.
The first instance of the "(italics mine)" form of acknowledging such additions in Ngram Viewer's search results is in M.L. Knapp, Discovery of the Cause, Nature, Cure and Prevention of Epidemic Cholera (1855):
Dr. Curran, (Dublin Quarterly Journ., Aug. 1847,) noticed in his practice that—
"A diseased state of the gums was one of the most constant symptoms, being absent in four cases only."—[Italics mine.]
The next-oldest instance of "(italics mine)" is from Transactions of the American Ophthalmological Society in 1873, but some two dozen instances appear from the period between 1882 and 1899. Most of these are from medical publications with a smattering from religious tracts, scholastic works, masonic journals, and elsewhere.
The form "(italics added)" debuts in Ngram Viewer results in 1827, in Report of the Commission Appointed by the Act of April. 21, 1825, to Revise the Statute Laws of This State [New York]:
§ 3. No person who shall have been convicted within this state, of an infamous crime, at any time previous to an election, shall be permitted to vote thereat; unless he shall have been pardoned by the executive, and by the terms of such pardon, restored to all rights of a citizen. [Act of 1822, § 25, italics added.]
Nineteenth-century use of this formulation is quite sparse and almost entirely limited to the context of law—a circumstance that I find somewhat surprising, given that the passive voice of that wording doesn't explicitly address the question of who added the italics to the quotation in question. This form of notation became much more popular after the turn of the twentieth century, though initially some of the impetus seems to have come from authors who were willing to go into self-defeating contortions such as "[italics added by writer]" to avoid using the dread first-person singular. Is the "writer" in such a case the original writer or the quoting writer?
The earliest definite instances of "(emphasis added)" that I could find in the Ngram Viewer results are in a 1919 casebook called Decisions of the Department of the Interior in Cases Relating to the Public Lands (where the editor twice uses the notation "[Emphasis added.]" following a block quote containing italic wording) and in People v. Woodcock, a 1921 California state appellate court opinion in which Judge Nourse, writing for a unanimous three-judge court uses the parenthetical phrase "(Emphasis added here.)" twice. Most of the other early (pre-1960s) instances of the "(emphasis added)" form are likewise from U.S. judicial opinions or other legal materials.
The form "(emphasis mine)" is several decades older than "(emphasis added)" in the Ngram Viewer search results. Interestingly, the earliest instance there (in The Overland Monthly, September 1889) indicates the introduced emphasis with large and small capital letters, instead of italics:
The Supreme Court [emphasis mine] say in 76 California Supreme Court Reports, page 360: "...The provisions of the act relative to the condemnation of private proerty, land, water, etc., for the uses prescribed therein, are in harmony with the Constitution and State laws, and in strict consonance with the views of the Supreme Court in Lux vs. Haggin, 69 Cal. 302. . . . THE ACT UNDER DISCUSSION IN ALL RESPECTS COMPLIES WITH THE VARIOUS PROVISIONS OF THE STATE CONSTITUTION."
In this instance, the author may have resorted to using capitalization to signify the added emphasis because italics already appeared (in accordance with standard legal style of the time) in the case name Lux vs. Haggin.
The other side of the coin involves quoters' pointing out when italicized wording did appear in the original writing as well as in the quoted version. The earliest legitimate dates in Ngram Viewer for "(emphasis in original)" and "(emphasis in the original)" are difficult to pin down because most of the early instances are either recently updated (and annotated) versions of older publications or are snippet views; however, it doesn't seem to go back much further than the late 1920s. A surprising number of the instances of "(emphasis in original)" during the period from the early 1930s to the 1950s appear to be from Communist (and Anti-Communist) literature.
In the nineteenth century, the most common short form for indicating that the original author of a quotation provided the italics was "(italics in original)." The earliest example in Ngram Viewer search results appears in "Poetical Translations of Faust," in Blackwood's Edinburgh Magazine (February 1840):
With regard to his own translation he [Job Crithannah] speaks thus. "I have proposed to myself to give the meaning of my author fully, neither skipping over, nor avowedly leaving out any part ... I have, therefore, considered it better on such occasions to give a good liberal English equivalent rather than a cramped verbality, so that the verse might flow, [italics in original,] without which no poetical vision could ever become agreeable to the English Reader, or approach to a display of Goethe's versification."
But "(italics in the original)" was also common and just as early, as in this excerpt from F.C. Brown, Letters to and from the Government of Madras (October 1838):
Persons dismissed from the office of Head-Sheristador, whose dismissal has been confirmed by the board of Revenue, are at liberty to appeal through the Board to Government. Petitions direct to Government will not be received. (italics in the original.)
Going back to 1811, the same year that Letters to John Aikin, M.D. appeared, the author of a book review published in The Edinburgh Review of Robert Wilson's Brief Remarks on the Character and Composition of the Russian Army added footnotes after quotations containing words that the original author had italicized, noting "*So printed in Italics in the original," and "*Italic in original."
One novel approach to indicating that someone other than the original author has added italics to a quotation appears in John Jackson, "Clinical and Physiological Researches on the Nervous System," from The Lancet (1873):
Laycock says ("Mind and Brain," Vol. ii, p.144)—"The functions of the hemispherical ganglia, as the organs of thought and mental action proper, are in unity with all the processes of life whatever, whether they be termed vegetative or animal" (no italics in original)."
Though that wording didn't catch on with other writers and editors, it expresses the same meaning as "(emphasis added)."