Aaargh - ya I knew there was a pending sale, I just never checked to see if it was finalized - just checked and it was in January, so the Seligs who bought the Brewers in 1970 are no longer the longest serving owners.
Steinbrenner bought the Yanks back in '73, making it him.
Next closest: Giles in Philly and Reinsdorf in Chicago, both in 1981.
So, the REAL answer is: Wrigley and Steinbrenner.
Guess I'll have to double check my facts next time!