The incident happened at the Preuss Station Post Office at La Jolla Avenue and Pico Boulevard in South Carthay.
Police said the gunman tried to force his way into the post office just after 8 p.m. Monday and a struggle ensued.
The manager was able to get the gun away from the intruder, but not before being struck once in the hand.
The attacker fled the scene and remains at large. Anyone with information was asked to call Los Angeles police at (877) LAPD-24-7, that's (877) 527-3247.