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 1841 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.

In Concord he devoted himself to poetry and chopping wood. He was befriended by Henry David Thoreau, and praised and often published in The Dial by Ralph Waldo Emerson. Some speculation identifies him as the "Poet" of Thoreau's Walden; the two were frequent walking companions.

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.

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".

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.

Death

Channing died December 23, 1901 in Concord. He is buried at the Sleepy Hollow Cemetery, Concord on Author's Ridge directly facing his longtime friend Thoreau.

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". 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."