The American, who triumphed in the 3M Open a week prior, was rarely troubled in the final round in Detroit.
Finau split six birdies with a single bogey as he finished the tournament on 26 under par.
His score was well-clear of runners-up Patrick Cantlay, Cameron Young and Taylor Pendrith, who were all tied on -21.
Speaking to reporters after his victory, Finau said: "What a wonderful couple of weeks it's been.
"I won the golf tournament last week and for some reason I left - bogeying the 18th hole - I had kind of a sour taste in my mouth.
"I think it just gave me extra motivation to put myself there again this week and just prove to myself the type of champion that I am."
England's Matt Wallace was the best of the British contingent, finishing equal 10th on 16 under.