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.
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. In 1950, he was a panelist during the first year of What's My Line?.
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.