Something tells me you rushed into that conclusion.
Declan
At least he didn’t say Randy Bachman and Fred Turner. He would have gone into overdrive.
I came into the thread to post that.
He directed the two most successful films in history.
He also directed the best of the “Alien” series and the first two terminators.
He helped start the current 3D cinema craze.
I can’t see how any other Canadian comes close.
See I was thinking John Kenneth Galbraith.
That would be a bad argument. The first college football game between Rutgers and Princeton was in 1869. New Brunswick New Jersey is the birthplace of football.
Joni Mitchell. Or Dayglo Abortions. Just can’t decide.
That sounds about right. 
My first thought was Joni Mitchell.
I’d say Lorne Michaels had more of an impact than Joni Mitchell (or Neil Young or Rush) - not everybody in America was a fan of those artists; you’d be hard-pressed to find someone who hasn’t seen SNL or at least one of the hundreds of movies made by SNL alumni.
None of those people will be in history books 100 years from now. Not even Canadian history books. They may have had influence on POP culture, but they had none whatsoever on any nation’s REAL culture, like an intertationally influential politician.They are already being forgotten, like Randy Bachman or Joni Mitchell.
A big stretch! Then again, she was from Massachusetts. She was a traitor to her homeland. 
Okay, I’ll give you Frederick Banting, one of the discoverers of insulin.
What? I get nothing for Galbraith? 
Okay, Galbraith, too. But not Neil Peart. This isn’t Cafe Society.
I’m still trying to decide: if a person discovered or invented a thing, and that thing has had a great effect on American culture, to what extent can we say that the person has influenced American culture? If you allow that sort of answer, it’s theoretically possible for a person to be “most influential in American culture” without ever having heard of America, which doesn’t seem right.
It’s the Canadians who dominate the entertainment industry!
I really don’t think politicians are what anyone, much less the OP, was thinking about when talking about CULTURE. The OP is clearing asking for people who directly impacted matter of culture - letters, arts. cinema, drama, sports, and music.
James Doohan, hands down
Mr Shatner is a nice guy, but I don’t know a single person in the sciences who’d have rather been Kirk than Scotty. Do you have a smart phone? Somewhere along its design was a geeky engineer who took one look at it and said,
“Whats the matter with that thing?”
“Its a power drain somewhere…”
examines
“Its in the main circuitry. I’ll get it…” 
Then you mean Pop Culture. Are you going to move this thread to Cafe Society?
Oooops! Missed the ;).