M*A*S*H, the TV series, was developed by Larry Gelbart which was adapted from the 1970 movie MASH, which was based on a 1968 novel MASH: A Novel About Three Army Doctors by Richard Hooker.
M.A.S.H. the TV series was based on MASH the film which was based on the novel MASH: A Novel About Three Army Doctors written by Richard Hooker.
However, if you mean MASH as in mashed potato, then I'm certain that it was serendipitously invented/created when some unknown person boiled the potatoes too long, probably a few hundred years ago.
Cherry Mash was created in 1918.
Monster Mash was created in 1997.
Zony Mash was created in 1976-07.
Mash Off was created on 2011-11-15.
Mash Confusion was created on 1999-08-24.
MASH - film - was created on 1970-01-25.
The Original Monster Mash was created in 1962.
Mash Out Posse - album - was created on 2004-05-25.
Mash Down Babylon Records was created in 2005.
The Big Mash Up was created on 2011-10-14.
Mash-Up - Glee - was created on 2009-10-21.
Live in Seattle - Zony Mash album - was created in 2002.