Conlan umpired in the National League from 1941 to 1965, officiating in five World Series (1945, 1950, 1954, 1957 and 1961) and six All-Star Games (1943, 1947, 1950, 1953, 1958 and the first 1962 contest). He also umpired in the playoff series to decide the NL's regular-season champions in 1951, 1959 and 1962 (some sources erroneously credit him with umpiring in the 1946 NL playoff as well). He was the home plate umpire when Gil Hodges hit four home runs on August 31, ; he also umpired in the April 30, game in which Willie Mays hit four home runs. He retired after the 1964 season, but returned to work as a substitute umpire for 17 games in 1965.