William Daniel Ehrhart was born on September 30th, 1948, in Roaring Spring, Pennsylvania, the third of four sons born to Rev. John H. Ehrhart, a Protestant minister, and Evelyn Marie (Conti) Ehrhart, who, after raising her children, became a special education teacher. Soon after Ehrhart was born, the family moved to Lewisburg, PA, and then in 1955 to Perkasie, PA, where John and Evelyn remained for the rest of their lives.

Immediately upon graduating from Pennridge High School in June 1966, Ehrhart joined the U.S. Marine Corps, serving three years, including 13 months in Vietnam. He subsequently earned bachelor's and master's degrees, and (at the age of 52) a doctorate. Over the years, he has held a wide variety of jobs, from merchant seaman to newspaper reporter to high school teacher. Through the 1990s, he made his living primarily as a writer and speaker, but early in the new millenium he returned to fulltime high school teaching at The Haverford School in suburban Philadelphia, PA, where he taught until retiring in June 2019. He remains active as a poet, writer, and speaker.

He has been married to the former Anne Senter Gulick since 1981, and their daughter Leela was born in 1986.

Ehrhart began writing when he was 15 years old, and has been writing more or less continuously ever since. His first published work, a poem about Swarthmore College, appeared seven years later in the Chronicle of Higher Education, and the following year eight of his poems were included in Winning Hearts and Minds: War Poems by Vietnam Veterans. Exclusively a poet until he was almost 30, he has since written and published a wide variety of nonfiction prose from 400-word newspaper commentaries to 40-page scholarly essays to 400-page personal narratives.

The influence of Ehrhart's encounter with the Vietnam War can readily be seen in his writing, but though he is known primarily as a "Vietnam War poet," in fact his subject matter ranges widely. He has written essays and articles on such topics as radio disc jockeys, tugboats on the Delaware River, the Internal Revenue Service, and a variety of modern and contemporary poets from William Wantling to Daniel Hoffman. His wife and daughter are major sources of inspiration for his poetry. His poems also reflect his respect for nature, his love of friends, his active engagement with the world around him, and his consternation at the human condition.