He portrayed Congressional custodian Andrew McNair in the Broadway version of 1776, which made him the one actor who stayed throughout the entire run of the show and was never understudied; he also played the part in the 1972 film of the musical. In the 1997 Broadway revival of 1776 Duell was a replacement member of the cast, filling the role of Caesar Rodney after Michael McCormick took on the role of John Adams. In 2010, he appeared in a one-night only staged reading of Evening Primrose by Stephen Sondheim.
He was born George william Duell in 1923 in Corinth, New York, to E. Janet (Harrington) and Leon George Duell, an employee of the International Paper Company. Sometime in his youth, his mother legally changed his name to Darwin william Duell. Duell never cared for his first name and thus always went by his middle name, william. Duell graduated from the Green Mountain Junior College (now Green Mountain College) (Vermont), Illinois Wesleyan University, and Yale University. A theater scholarship at Green Mountain College is named after him.
Duell married his wife, Mary Barto, in 2004. Duell died of respiratory failure in December 2011. He was 88.