Youmans was born into a performing family; his great-uncle was Broadway composer Vincent Youmans. William made his Broadway debut in 1981, playing Leo Hubbard in The Little Foxes. He performed numerous roles in the original production of Big River and sang the role of Alcindoro in the Baz Luhrmann-directed revival of La Bohème. He also portrayed Sir Richard Bingham in The Pirate Queen and performed in the play The Farnsworth Invention. In 1986 he was in the Off Broadway play The Widow Claire by Horton Foote. He played a few different roles in the musical Coraline. He originated the roles of Albert and Ben in the world premiere of The Secret Garden. In 1993, Youmans played several roles in the Off-Broadway recording of Alan Menken's Weird Romance.[1]
In 1997 he originated the role of John Jacob Astor in Titanic, and 2003, originated the role of Doctor Dillamond in Wicked. After a San Francisco tryout, in which the role was played by John Horton, the original Broadway production opened October 30, 2003, starring Kristin Chenoweth and Idina Menzel. Youmans left the role on July 24, 2005, and later reprised the role in the Chicago production for a limited engagement from December 12, 2006, through January 14, 2007. He can be heard on the show's original Broadway cast recording. He made a return on March 7, 2023.
In 2015, he originated the role to critical acclaim as The Director in Being Seen by Richard Gustin at the International Fringe Festival NYC.
In 2018, he played Bascombe and understudied the Starkeeper in the Broadway revival of Carousel. In November 2019, he joined the cast of To Kill a Mockingbird as Dr. Reynolds/Mr. Roscoe and understudy Link Deas and Sheriff Heck Tate. [2]