Channing was born in Boston, Massachusetts to Dr. Walter Channing, a physician and Harvard Medical School professor. He attended Boston Latin School and later the Round Hill School in Northampton, Massachusetts, then entered Harvard University in 1834, but did not graduate. In 1839 he lived for some months in Woodstock, Illinois in a log hut that he built; in 1840 he moved to Cincinnati. In the fall of 1842 he married Ellen Fuller, the younger sister of transcendentalist Margaret Fuller and they began their married life in Concord, Massachusetts where they lived a half-mile north of The Old Manse as Nathaniel Hawthorne's neighbor.
Channing wrote to Thoreau in a letter: "I see nothing for you on this earth but that field which I once christened 'Briars;' go out upon that, build yourself a hut, and there begin the grand process of devouring yourself alive. I see no alternative, no other hope for you." Thoreau adopted this advice, and shortly after built his famous dwelling beside Walden Pond. Some speculation identifies Channing as the "Poet" of Thoreau's Walden; the two were frequent walking companions.
In 1843 he moved to a hill-top in Concord, some distance from the village, and published his first volume of poems, reprinting several from The Dial. Thoreau called his literary style "sublimo-slipshod". The printing of a compilation of these poems was subsidized by Samuel Gray Ward.
In 1844–1845, Channing separated from his family and restarted his wandering, unanchored life. He first spent some months in New York City as a writer for the Tribune, after which he made a journey to Europe for several months. In 1846 he returned to Concord and lived alone on the main street, opposite the house occupied by the Thoreau family and then by Alcott. During much of this time he had no fixed occupation, though for a while, in 1855-1856, he was one of the editors of the New Bedford Mercury. After enumerating his various wanderings, places of residence, and rare intervals of employment, his housemate Franklin Benjamin Sanborn wrote of him:
In 1873, Channing was the first biographer of Thoreau, publishing Thoreau, the Poet-Naturalist.
When visiting the Emersons in 1876, the young poet Emma Lazarus met Channing and accompanied him on a tour of some of the places Thoreau had loved, stating in her journal in regard to the friendship between Thoreau and Channing, “I do not know whether I was most touched by the thought of the unique, lofty character that had instilled this depth and fervor of friendship, or by the pathetic constancy and pure affection of the poor, desolate old man before me, who tried to conceal his tenderness and sense of irremediable loss by a show of gruffness and philosophy. He never speaks of Thoreau's death, but always 'Thoreau's loss', or 'when I lost Mr. Thoreau,' or 'when Mr. Thoreau went away from Concord'; nor would he confess that he missed him, for there was not a day, an hour, a moment, when he did not feel that his friend was still with him and had never left him. And yet a day or two after, when I sat with him in the sunlit wood, looking at the gorgeous blue and silver summer sky, he turned to me and said: 'Just half the world died for me when I lost Mr. Thoreau. None of it looks the same as when I looked at it with him.' " Ellery Channing gave Emma Lazarus Henry Thoreau's compass.
Channing died December 23, 1901 in Concord, at the home of Franklin Benjamin Sanborn, where he had spent the final ten years of his life. He is buried at Sleepy Hollow Cemetery, Concord on Author's Ridge directly facing his longtime friend Thoreau. Frank Sanborn paid for Channing's burial plot.
In a July 19, 1902, Springfield Republican article, Frank Sanborn states, "This week the Channing lot in Sleepy Hollow cemetery received the ashes of the poet Ellery Channing, whose remains were cremated, at his request, last January, but not committed to earth till July 15. The only service was the reading above the grave of a Greek epitaph... The stanza written by Channing for such an occasion half a century ago was also read, with a slight change, adapting it to the stately pine trees that surround his burial place, exactly opposite the grave of his friend Hawthorne:
In a later Republican column, Sanborn informs, "I have lately come upon the Greek Iambics which I buried with the ashes of Channing in Sleepy Hollow cemetery; and I copy them here in English type, that they may not be wholly lost:--
"A word about the Greek. The first two lines mean: 'Here I bury your ashes in a small container,/ Dearest singer, whose songs live blossoming (i.e. blossom and live)'. The third line is prose: 'May the earth fall light upon you.' The verses are faintly reminiscent of the well-known epigram of Callimachus to Heraclitus."
Critic Edgar Allan Poe was particularly harsh in reviewing Channing's poetry in a series of articles titled "Our Amateur Poets" published in Graham's Magazine in 1843. He wrote, "It may be said in his favor that nobody ever heard of him. Like an honest woman, he has always succeeded in keeping himself from being made the subject of gossip". In this article, He mistakes W. Ellery Channing to be the son rather than nephew of William E. Channing and voices his views as 'we' (the Society in Baltimore) rather than 'I'. A critic for the Daily Forum in Philadelphia agreed with Poe, though he was surprised Poe bothered reviewing Channing at all. He wrote: "Mr. Poe, the most hyper-critical writer of this meridian, cuts the poetry of William Ellery Channing, Junior, if not into inches, at least into feet. Mr. C's poetry is very trashy, and we should as soon expect to hear Bryant writing sonnets on a lollypop as to see Mr. Poe gravely attempt to criticize the volume."
Nathaniel Hawthorne metaphorically appraised Channing's oeuvre as of particularly high quality, if uneven, in the short story "Earth's Holocaust".