From a 'M*A*S*H' wikia:
[Father] Mulcahy came from a rather large family [in Philadelphia] who weren't close, from things he revealed throughout the series. Both his parents drank, and displayed their temper on occasion; about the only thing he and his father had in common was an interest in boxing, and his father took him to fights. Mulcahy had to share a bed with his brothers, and the only family member he spoke fondly of was his sister Catherine. When given the chance to send greetings to his family during a filmed interview, he simply waved to the camera and said "Hello", adding nothing.
By 1961, one of Father Mulcahy's older brothers was living in Los Angeles where he worked as a building superintendent. Private investigator Paul Drake tricked him into opening up an apartment while he was in the guise of a TV repairman. But it was obvious that "Pops" was not taken in by the ruse.
'Perry Mason' - "The Case Of The Violent Vest"
Father Mulcahy - William Christopher
"Pops" (Mulcahy) - Bill Erwin
Since he was only addressed as "Pops", and listed in the credits as "Superintendent", we could probably find some other character played by Bill Erwin who was only listed by his first name and maybe situated in Los Angeles who could be the same character.
Does the name and maybe the face seem familiar to you? Here's how you probably best remember Bill Erwin......