Found this question about usage, but not origin.
Where did this phrase come from?
There is an early tantalizing suggestion in Letters Concerning the Present State of England (1772) that the private desk may be a special locus of inspired thought:
Population is said to be all in all, increase your people, and you need take no account of any other matter, population being the political thermometer ; but such notions, when they walk from the desk of the visionary to the cabinet of the statesman, are productive of infinite mischief. In numerous countries and cases, increasing the people is a political evil.
But ultimately the distinction that the anonymous author makes in this passage is between theory unattached to practical consequences ("the desk of the visionary") and policy imposed in the real world ("the cabinet of the statesman"). Still, the author feels that the personal writing desk is a strong enough metaphor for the private man of genius to stand for him without further elaboration.
Samuel Trumbull and Charles Miner
That early instance, however, is the only arguably relevant antecedent I could find to the two "from the desk of authors" that RaceYouAnytime mentions in a comment above—Beri Hesdin (Samuel Trumbull) and Poor Robert the Scribe (Charles Miner). The earliest notice I found of the former is from "It Is Not Good That Man Should Be Alone," in New-York Magazine or Literary Repository (August 1797), reprinted from the [Walpole, New Hampshire] Farmer's Weekly Museum:
From the Desk of Beri Hesdin.
"We have sought him, but he is gone! We wist not whither."
The good-natured readers of the Massachusetts Magazine, since its final departure, may have adopted this exclamation of the affectionate apostle; and in the fulness of their love for any thing original, sought inquiringly for Beri Hesdin. But, alas! such is human nature—such the depravity of man, that he is ever trying to escape those duties wherein the emolument and glory of self is not the primary object. To this charge, Beri Hesdin pleads guilty : he owns that, unmindful of his parochial office, he has lately continued strolling along the banks of his village stream; and while the cure of souls should have employed his contemplations, his thoughts became vagrant as himself, and with they chose rather to chase the bubbles on the surface of the water, than, from the changing garb of nature, cull the lessons of morality. ...
Conceive, ye strollers from your duty, how his conscience must have smote him, when he returned to his home and study, to find the following sermon on his desk—a sermon, couched in his native dialect, and bearing the mark of the preacher upon it; though, from the general tenor of the whole, he is rather suspicious it is the zealous labor of aunt Susan, a grey-haired maiden of the parish, who from the time she could convey an idea, became a strong advocate in favor of matrimony. She has not seldom been ever heard depicturing with liquorish tongue, to every femme sole, the most lovely scenes of a union of the flesh. Not long since, she published a scheme of a lottery, for the disposal of bachelors and misses of forty-two; together with a long treatise on the beauty of wrinkles: but we hasten to this choice sermon.
The remainder of the article—accounting for more than half of its total length—is devoted to a (not particularly religious) sermon that expresses vigorous sympathy for marriage, taking as its starting point the text drawn from Genesis, "It is not good that man should be alone." In short, the sermon that forms the core of the article comes "from the desk of Beri Hesdin" (where it was allegedly found) and yet is asserted not to be the work of Beri Hesdin.
Two subsequent pieces "from the Desk of Beri Hesdin" appear in The Weekly Magazine of Original Essays, Fugitive Pieces, and Interesting Intelligence, volume 2 (May 5–July 28, 1798). Like the previous piece, they take the form of sermons or soliloquys on subjects suggested by a biblical verse—in one case "Israel is an empty vine; he bringeth forth fruit unto himself," and in the other "O! pity me—for I am poor; have compassion on me—for my cruse of oil faileth." The author does not pretend to have found these sermons, like the first one, on his desk, writ by another hand; but he does continue the practice of introducing them with the the phrase "From the Desk of Beri Hesdin."
Although Charles Miner, writing as "Poor Robert the Scribe" uses the same trick of attribution that Trumbull did—using the attribution "from the desk of Poor Robert the Scribe" in place of a standard byline, he does not use the same sermon-based mode of essay writing; rather, he presents a vignette or anecdote that points to a moral lesson expressed in an aphorism or proverb.
In Essays from the Desk of Poor Robert the Scribe: Containing Lessons in Manners, Morals, and Domestic Economy (July 1815), we learn that Miner wrote most of his pieces during the four years from 1810 through 1813. A number of these articles appear in The Lady's Miscellany, or Weekly Visitor (1811–1812), always under the heading "From the Desk of Poor Robert the Scribe" The most famous of these is the article "Who'll Turn the Grindstone?" a piece that appeared initially in the Luzerne [Pennsylvania] Federalist (September 7, 1810) and seems to have been the source of the idiomatic expression "have an axe to grind" (see Origin of "to have an axe to grind"). In a letter of July 17, 1859 to the Norwich, Connecticut Jubilee celebration, Miner explicitly acknowledged his debt to Trumbull for the wording "From the Desk of":
Samuel Trumbull, the oldest son, was a young man of a good deal of reading, and of ready wit. He wrote several essays under the head of "From the desk of Beri Hesden." The hint and the name of the essays—"From the desk of poor Robert the Scribe," I am sure I owed to him.
The likeliest explanation for the origin of the phrase "from the desk of X" lies in the literary conceit of Beri Hesdin (Samuel Trumbull), in 1797 that an anonymous sermon had appear on his desk to be shared with the world. This is actually a less roundabout tactic than the one that Charles Dickens initially used in 1840 when he presented The Old Curiosity Shop in its originally serialized form in his periodical Master Humphrey's Clock—as a story narrated by a third party who had learned of the events during his rambles about London.
Still, the continued popularity of "from the desk of"—to the extent that it remains embedded in English long after people forgot its origins—owes a large debt as well to the efforts of Charles Miner, writing a dozen years after Trumbull, "from the desk of Poor Robert the Scribe."