Born in St. Louis, Missouri, Seidel graduated from St. Louis Country Day School and earned his A.B. at Harvard University in 1957. He now lives in New York City.

In 1962, his first book, Final Solutions, was chosen by a jury of Louise Bogan, Stanley Kunitz, and Robert Lowell for an award sponsored by the 92nd Street Y, with a $1,500 prize. However, both the Y and the publisher rejected the manuscript for a variety of reasons; one being that, in their opinion, "matter in one of the poems libeled a noted living person";and another being that the national head of the YMCA expressed concern that some of the poems were "anti-Semitic and anti-Catholic," a claim Seidel considered preposterous. Seidel refused to make requested changes, and the three judges quit when the prize was withdrawn and the publisher reneged.Random House published the book the following year, but seventeen years would pass before Seidel published another work. His collection, The Cosmos Poems, was commissioned by the American Museum of Natural History to celebrate the opening of the new Hayden Planetarium in 2000.

Seidel is independently wealthy-- he inherited rather than earned his personal fortune-- and is fascinated by Ducati motorcycles, of which he owns four.

His second book, Sunrise, was the 1980 Lamont Poetry Selection. His book Going Fast was a finalist for the 1999 Pulitzer Prize for Poetry. He won the PEN/Voelcker Award for Poetry in 2002. His collection Ooga-Booga was a finalist for the National Book Critics Circle Award and was shortlisted for the 2007 International Griffin Poetry Prize. A limited run of his most recent collection of new verse, Evening Man was published in 2008. The following year saw the publication of the career-spanning anthology Poems:1959-2009.