The M in John M Jackson does not stand for anything. John was forced to add the M to distinguish himself from other John Jacksons when he registered with the Screen Actors Guild.
John played football throughout high school. He joined the football team when he attended the University of Texas, but a knee injury ended any hopes of a career as a football player.
Years before he became a part of the JAG cast, John played Captain West in A Few Good Men (1992). This was only a minor role, but he took part of the character into his latter role as Admiral Chegwidden in JAG.
Chuck Carrington: John M. Jackson is a 6'3", 50-year-old bald man who acts like an 8-year-old. He is a comedian. He's always the center of attention. He's a big kid, and for me it's been great. Almost every scene I have done is with him. He has a great personality, and he's a joy for all of us to work with.
John M. Jackson: I can honestly say that my son is a friend of mine. That's a hard thing to do. It's like teaching. You want to get a good rapport with your students, but there's a line that can't be crossed. If you do cross it, the roles get gray and you don't have the same ability to teach.