Mitchell, the grandson of politician and jurist Stephen Mix Mitchell, was born in Norwich, Connecticut. He graduated from Yale College in 1841, where he was a member of Skull and Bones and studied law, but he soon took up literature. Throughout his life he showed a particular interest in agriculture and landscape gardening, which he followed at first in pursuit of health. He served as U.S. consul at Venice, Italy, from 1853 to 1854, and in 1855 he settled at his estate, called Marvelwood, near New Haven, Connecticut.

He was best known as the author (under the pseudonym of "Ik Marvel") of the sentimental essays contained in the volumes Reveries of a Bachelor, or a Book of the Heart (1850) and Dream Life, a Fable of the Seasons (1851). The latter book was dedicated to Washington Irving, to whom Mitchell was introduced by Lewis Gaylord Clark. Irving said of the dedication: "Though I have a great disinclination in general to be the object of literary oblations and compliments... I have enjoyed your writings with such peculiar relish and have been so drawn toward the author by the qualities of head and heart evinced in them, that I confess I feel gratified by the dedication". Mitchell produced books of travel, volumes of essays on rural themes, including My Farm of Edgewood: A Country Book (1863), sketchy studies of English monarchs and of English and American literature, and a character novel entitled Doctor Johns (1866). His other works include About Old Story-tellers (1878) and American Lands and Letters (1897-1899).

Oliver Wendell Holmes, Sr., called him "one of the pleasantest of our American writers."