Following his major league debut in September 1966, Monday began the next season in the majors, the A's last year in Kansas City. The team moved west to Oakland prior to the 1968 season, his first as an All-Star. Monday was with the A's through 1971, their first as American League West champions. He was traded for pitcher Ken Holtzman that November, and spent five productive seasons with the Chicago Cubs. In January 1977, Monday was traded in a five-player deal to the Los Angeles Dodgers for Bill Buckner and Iván DeJesús. The Dodgers won the National League pennant in 1977 and 1978.