Untermeyer was born in New York City. He married Jean Starr in 1906. Their son Richard was born in 1907 and died under uncertain circumstances in 1927. After a 1926 divorce, they were reunited in 1929, after which they adopted two sons, Laurence and Joseph. He married the poet Virginia Moore in 1927; their son, John Moore Untermeyer (1928), was renamed John Fitzallen Moore after a painful 1929 divorce.

In the 1930s, he divorced Jean Starr Untermeyer and married Esther Antin. This relationship also ended in divorce in 1945. In 1948, he married Bryna Ivens, an editor of Seventeen magazine.

He was known for his wit and his love of puns. For a while, he held Marxist beliefs, writing for magazines such as The Masses, through which he advocated that the United States stay out of World War I. After the suppression of that magazine by the U.S. government, he joined The Liberator, published by the Workers Party of America. Later he wrote for the independent socialist magazine The New Masses.

He was a co-founder of "The Seven Arts", a poetry magazine that is credited for introducing many new poets, including Robert Frost, who became Untermeyer's long-term friend and correspondent.

In 1950, he was a panelist during the first year of What's My Line?. According to Bennett Cerf, Untermeyer would sign virtually any piece of paper that someone placed in front of him, and Untermeyer inadvertently signed a few Communist proclamations. According to Cerf, Untermeyer was not at all a communist, but he had joined several suspect societies that made him stand out. He was named during the hearings by the House Committee on Un-American Activities investigating communist subversion. The Catholic War Veterans and "right wing organizations" began hounding Mr. Untermeyer. Goodson-Todman held out against the protests of Untermeyer for some time, but finally veterans began picketing the theater. The pressure became too great, and the sponsor Jules Montenier said, “After all, I'm paying a lot of money for this. I can't afford to have my product picketed.” At that point, the producers told Untermeyer that he had to leave the program. This action led to Bennett Cerf becoming a permanent member of the program. The controversy surrounding Untermeyer led to him being blacklisted by industry.

He was the author or editor of close to 100 books, from 1911 until his death. Many of them and his other memorabilia are preserved in a special section of the Lilly Library at the Indiana University.

Schools used his Modern American and British poetry books widely, and they often formed students' introduction to poetry. He and Bryna Ivens Untermeyer created a number of books for young people, under the Golden Treasury of Children's Literature. He lectured on literature for many years, both in the US and other countries.

In 1956 the Poetry Society of America awarded Untermeyer a Gold Medal. He also served as a Poet Laureate Consultant in Poetry to the Library of Congress from 1961 until 1963.

Poetry

• "Burning Bush"
• "Challenge"
• "Coal Fire"
• "Disenchanted"
• "First Love"
• "Last Words Before Winter"
• "Leviathan"
• "Long Feud"
• "Portrait Of A Machine"
• "Questions At Night"
• "Scarcely Spring"
• "Summer Storm"
• "Thanks"
• "The New Adam"
• "These Times"

Parodies

• Heavens
• Including Horace
• ... And Other Poets (1916)
• Collected Parodies (1926)

Autobiography

• From Another World (1935)
• Bygones (1965)

Essays

• American Poetry Since 1900 (19??)
• The Forms Of Poetry (1926)
• Play in Poetry (1938)
• Doorways to Poetry (1938)
• The Lowest Form of Wit (1947)
• The Pursuit of Poetry (1969)

Critical collections

• The Poems of Henry Wadsworth Longfellow (1943)
• The Poetry and Prose of Walt Whitman (1949)
• The Letters of Robert Frost to Louis Untermeyer (1963)
• The Love Poems of Elizabeth and Robert Browning (1994)
• The Love Poems of Robert Herrick and John Donne (1948)

Fiction

• Moses (1923)
• The Fat of the Cat and Other Stories
• The Donkey of God and Other Stories (1932)
• The Kitten Who Barked (1962), illustrator: Lilian Obligado
• The Second Christmas (1964), illustrator: Louis Marak
• Cat O' Nine Tales (1971), illustrator: Lawrence DiFiori
• The Dog of Pompeii

Biography

• Heinrich Heine: Paradox and Poet (1937)
• Makers of the Modern World (with John Moore (1955)
• Makers of the Modern World selections, Japanese translation (1971)

Anthologies

• Modern American Poetry (1919) (6th edition, 1942)
• Modern British Poetry (1920) (5th edition, 1942)
• Modern American and British Poetry (1919)
• Yesterday and Today (1926)
• New Songs for New Voices (1928), with Clara and David Mannes, illustrator: Peggy Bacon
• A Treasury of Great Poems (1942, 1955)
• The Golden Treasury of Poetry (1959), illustrator: Joan Walsh Anglund
• Story Poems (1946, 1972)
• Early American Poets (1952)
• An Uninhibited Treasury of Erotic Poetry (1963)
• A Galaxy of Verse (1978)
• Men and Women: the Poetry of Love (1970), illustrator: Robert J. Lee
• Collins Albatross Book of Verse (1933, 1960)
• Stars To Steer By (1941)
• Lots of Limericks (1961), illustrator: R. Taylor

• The Book of Living Verse (1932, 1945)
• Rainbow in the Sky (1935), illustrator: Reginald Birch
• A Treasury of Laughter (1946)
• An Anthology of New England Poets (1948)
• The Best Humor of 1949-1950 (with Ralph E. Shikes, 1950)
• The Best Humor Annual (with Ralph E. Shikes, 1951)
• The Best Humor Annual (with Ralph E. Shikes, 1952)
• The Magic Circle (1952)
• A Treasury of Ribaldry (1956)
• The Britannica Library of Great American Writing (1960)
• Big and Little Creatures (1961), with Bryna Ivens Untermeyer
• Beloved Tales (1962), with Bryna Ivens Untermeyer
• Old Friends and Lasting favorites (1962), with Bryna Ivens Untermeyer
• Fun and Fancy (1962), with Bryna Ivens Untermeyer
• Creatures Wild and Tame (1963), with Bryna Ivens Untermeyer
• The Golden Book of Poems for the Very Young (1971)
• A Treasury of Great Humor (1972)

Adaptations and translations

• Poems of Heinrich Heine (19??)
• The Wonderful Adventures of Paul Bunyan (1946), illustrator: Everett Gee Jackson
• More French Fairy Tales (1946), illustrator: Gustave Doré
• Cyrano de Bergerac (1954), illustrator: Pierre Brissaud
• Aesop's Fables (1965), illustrator: A. and M. Provensen
• Songs of Joy from the Book of Psalms (1967), illustrator: Joan Berg Victor
• Tales from the Ballet (1968), illustrator: A. and M. Provensen
• A Time for Peace (1969), illustrator: Joan Berg Victor
• The World's Great Stories (1964)
• The Firebringer (1968)
• Lines to a Pomeranian Puppy Valued at $3500 (1950), musical adaptation of Untermeyer poem by Irving Ravin