DiMuro was born in Brooklyn, New York, and attended Henry Snyder High School in Jersey City, New Jersey, where he played football, and then Jersey City Junior College. He served in the Air Force in West Germany, but a broken finger in the service ended his baseball playing career and he turned to umpiring, graduating from the Al Somers School and working in the Kentucky–Illinois–Tennessee League (1955), Northern League (1956), Eastern League (1957) and International League (1958–62). He joined the AL staff and immediately worked a full schedule as a rookie in 1963. He was selected to work the World Series in 1969 and 1976, the All-Star Game in 1965, 1967, 1972 and 1981, and the American League Championship Series in 1971, 1975 and 1978, serving as crew chief in 1978.