South African Clark recovered from a bogey at the opening hole to card five-under-par 65 at Royal Montreal.
He finished on 17-under 263, notching just his second victory on the PGA Tour in which he has 12 runner-up placings.
Third round leader Furyk shot 69 for second place on 16-under.
He missed a 15-foot birdie putt at the final hole, continuing a frustrating run of near-misses since his 16th and most recent tour victory nearly four years ago.
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