Na continued his mastery of the classic course in Fort Worth, Texas, starting the day with a two-shot advantage that was never really challenged.
Na's six birdies included a 33-footer at the fourth and a 20-footer at the eighth.
He rolled in a 12-footer at the 72nd hole to cap his four-under par round with a birdie for a 13-under total of 267.
"It feels great," said Na, who celebrated with his toddler daughter, Sophia, in his arms.
Na, 35, boasts an impressive record at Colonial, where he thrust himself into contention with a second-round 62. That was his third score of 62 or better on the course, where he matched the record of 61 on the way to a fourth-place finish last year.
He said he eyed the club's "wall of champions" on the first tee Sunday, "and in my head I engraved my name on it.
"Every year I come here I know I had a chance to win and I knew before my career was over I was going to definitely win here," added Na, who believes the compact course is one of "seven or eight" that offer him a chance to win on the US tour without a big-hitting game.
It was the second win in less than a year for the 35-year-old Na, a Las Vegas resident who was born in South Korea but came to America when he was just 8 years old.
He last won at the 2018 Greenbrier event in July, and his best finish this season prior to Fort Worth was a tie for fifth at the WGC Match Play in March.
Na was also given a 1973 fully restored blue Dodge Challenger as one of his perks for winning the tournament which he quickly turned over to his caddie Kenny Harms.
On Tuesday before the tournament started, Harms asked Na if he could have the car if they won the tournament. Na agreed.
"I don’t know how my caddie convinced me to give him the car, but he’s a good salesman, I guess. He sold me into it. But I’m more than happy to give it to him. He deserves it.
"I got something cooler right here," said Na holding his trophy.
Finau, who was among a group of five players to start the round two shots behind Na, moved within one shot of the leader with his birdie at the second.
He was two adrift on the back nine but the gap widened when Na birdied 14 and Finau bogeyed 16.
Finau finished with four birdies and two-bogeys in a two-under 68 for 271.
It was a further stroke back to American Andrew Putnam (66) and Taiwan's Pan Cheng-tsung (69).
Local favorite Jordan Spieth, who also started the day two off the lead, couldn't maintain his hot streak on the greens.
He closed with a two-over par 72 that left him eight back and still in search of his first victory since he captured his third major championship at the British Open in 2017.