Giles Goat-Boy, which was published in 1966, by John Barth is an early example, though perhaps not the earliest, of a novel that consciously uses the Hero's Journey as a structuring principle.
In his essay, "Mystery and Tragedy: The Twin Forces of Ritual Heroism", Barth remarks how he studied comparative mythologists Otto Rank and Joseph Campbell, after a reviewer suggested that his novel, The Sot-Weed Factor was a "parodic implementation" of Lord Raglan's "Wandering Hero Myth." Barth read Rank's Myth of the Birth of the Ritual Hero and Campbell's The Hero with a Thousand Faces in preparation for his novel, Giles Goat-Boy (Clavier 166).
Giles Goat-Boy features a miraculous birth (George Giles is a boy raised as a goat). He has some revelations about the truth of his birth, which is his call to adventure, and he faces many trials on his way to New Tammany College.
Even earlier perhaps, relative to The Hero with a Thousand Face's 1949 publication date, is William Gaddis's The Recognitions from 1955, which, in the introduction to the Dalkey Archive edition, William Gass compares its structure to the Hero's Journey, monomyth, or urmyth. Though, I cannot find an explicit reference that Gaddis has made regarding consciously using the Hero's Journey to structure his novel, he did read James George Frazier's The Golden Bough during the composition of The Recognitions, which is known to have been influential on Joseph Campbell as well.
Critic, Steven Moore also makes a similar argument to Gass's about The Recognition's monomythic structuring in the foreword to his reader's guide to the novel, but I do not have the source available at the moment.
The case for Giles Goat-Boy's indebtedness to the Hero's Journey is stronger than The Recognition's, but there may be an even earlier text that more explicitly references the Hero's Journey that I am unaware of.