His contributions provided elementary infrastructure used in algebra, geometry, topology, number theory, and logic.

His most famous result is his proof, joint with John G. Thompson, of the Feit–Thompson theorem that all finite groups of odd order are solvable.

It was often the case that, while the conclusions concerned groups of complex matrices, the techniques employed were from modular representation theory.

"In October 2003, on the eve of Professor Feit's retirement, colleagues and former students gathered at Yale for a special four-day "Conference on Groups, Representations and Galois Theory" to honor him and his contributions.

"[5] He died in Branford, Connecticut in 2004 and was survived by his wife, Dr. Sidnie Feit, and a son and daughter.