He is an American actor . He is most well known for playing Father Mulcahy on the television series M \* A \* S \* H. He also played Private Lester Hummel on Gomer Pyle , U.S.M.C. His wife 's name is Barbara O'Connor ; together they wrote a book , Mixed Blessings , about their experiences raising their autistic son . William Christopher ( born October 20 , 1932 ) is an American actor who is best known for playing Father Mulcahy on the television series M \* A \* S \* H and Private Lester Hummel on Gomer Pyle , U.S.M.C.