Reaching the major leagues after umpiring in the Pacific Coast League from 1965 to 1973, Rennert umpired in six National League Championship Series (1977, 1981, 1982, 1986, 1988, 1990), two All-Star Games (1979, 1984), and three World Series (1980, 1983, 1989); he was behind the plate when the Oakland Athletics won the 1989 World Series.