If Quebec had succeeded in becoming independent,
Camille Laurin could have ended up as president or prime minister or whatever, I suppose. He was the guy behind Bill 101, a pivotal piece of legislation, though going by wiki, he didn't hold a cabinet position.
Sorry that that depends on what some might consider to be the Canadian constitutional equivalent of Sealion, but off the top of my head, I can't think of any other prominent politicians who were clinical psychologists.