He played 14 seasons in Major League Baseball (MLB) from 1977 to 1990 for the Los Angeles Dodgers, Houston Astros, San Francisco Giants, Milwaukee Brewers, and Seattle Mariners.

He got 60 scholarship offers for football, five for basketball and none for baseball, where he played shortstop and twice hit two home runs in one inning.

While playing in the minor leagues in the middle of the 1981 season Max Venable and Guy Sularz gave Jeffrey Leonard the nickname "HackMan"[citation needed] because he had a habit of swinging at the first pitch no matter what.

[citation needed] He also led the Pacific Coast League in hits (183) and batting average (.365) while playing for the Albuquerque Dukes in 1978.

[citation needed] Leonard, along with Dave Bergman, was traded from the Houston Astros to the San Francisco Giants for Mike Ivie on April 20, 1981.

[5] Jeffrey Leonard's greatest moment as a player most likely occurred during the 1987 National League Championship Series while with the San Francisco Giants.

Leonard was among a group of players that Ueberroth cited as having had a "prolonged pattern of drug use" and had involvement in cocaine distribution; these players were issued one-year suspensions from Major League Baseball, however, the suspensions were to be waived in exchange for donations to drug-treatment programs, and community service performed with drug-related organizations.

[11] The Pittsburgh drug trials are considered one of Major League Baseball's biggest scandals of all time, albeit one that was "behind the scenes" and did not affect play on the field.