On August 5, 1999 — Mark McGwire hits his 500th career home run off Padres’ hurler Andy Ashby, becoming the 16th major leaguer and the fastest to reach this milestone. The St. Louis slugger is first to accomplish the feat a year after reaching the 400 home run plateau.

Source:
Baseball Reference August 5
National Pastime
Retro Sheet
Hall of Fame
SABR Games Project
Replay The Game