Kirk was also expecting a playoff. He just thought it would be with playing partner Brandt Snedeker and not Spieth, the local Masters champion who was within one stroke of Kirk's lead after draining a 20-foot birdie at No. 18 when finishing two groups earlier.
In the end, there were no extra holes Sunday.
Kirk got the plaid jacket that goes to the winner at Hogan's Alley, and Spieth finished as a runner-up in Texas for the third time this season.
After an errant tee shot at the last hole, Kirk made with a clutch 7-foot par putt for his fourth PGA Tour victory. Snedeker's 12-foot birdie try to tie the lead had just slid by the cup.
"I had the mentality I was getting ready to get up and make that putt to get into a playoff. That was what I thought," Kirk said. "Close friends with Brandt and play a lot of golf with him, and he doesn't miss very many of those putts. ... Once he didn't make it, then I was able to change my mindset a little bit."
His closing 4-under 66 got Kirk to 12-under 268, a stroke ahead of Snedeker (67), Spieth (65) and Jason Bohn, whose final-round 63 included six birdies in a row on the front nine.
Spieth, the 21-year-old Masters champion from Dallas, was making a bid to win the first of consecutive tournaments at home in the North Texas.
Just like the rest of the week, Spieth got a rousing ovation when he got to the 18th green. It got even wilder after he sank the long birdie.
"I wanted to give them something to cheer about," Spieth said. "It was nice to have that one just to give myself an outside chance."
Spieth, whose near-birdie at the par-3 16th hole Sunday became a bogey instead, had consecutive runner-up finishes in his home state before winning last month at Augusta. He finished behind fellow Texan Jimmy Walker at San Antonio and lost in a three-man playoff at Houston.
At Colonial, Spieth made a birdie at No. 18 to end the first round with a 64 and in a four-way tie for the lead. But he shot 73 in the second round Friday.
The PGA Tour is next at the Byron Nelson Championship in Irving, where Spieth twice made the cut as a teenage amateur.
"I can look back at the runner-up finishes this year, and they were normally good weeks the next week, so I can take confidence into that," he said. "I can't take it for granted though."
After Kirk hooked his tee shot at No. 18 into the left rough, Bohn and Spieth went to the nearby No. 1 tee to hit a few balls.
Spieth had made his final putt only a few minutes earlier. Bohn finished more than an hour before that, when a 28-footer lipped the cup and left him lifting the putter over his head in frustration.
"I thought it was in, that's why I started running," Bohn said. "I think it just straightened out."
Kirk hit his approach from 155 yards over the green. Snedeker, after a similar tee shot, put his in position for a possible birdie.
"I wanted to get myself in contention to give myself a chance. I did that," said Snedeker, whose six birdies all came in the first 11 holes, when he also had two bogeys. "Failed miserably on the exam though."
After Saturday's best round with a 65, Kirk started the final round with an eagle. His only bogey came after hitting his drive at No. 7 into the rough.
The $1.17 million check will be plenty to cover the cost of the new home he is buying close to the University of Georgia, where as a senior in 2007 he won the Hogan Award given by Colonial to the nation's top college player.
Kevin Na, the outright leader after the second and third rounds, shot 72 and finished in an eight-way tie for 10th at 9 under. He was among the quartet tied for the first-round lead.
A 54-hole leader hasn't won Colonial since Phil Mickelson in 2008.
"The back nine, I gave myself some chance to make birdie, and I just made nothing," Na said. "Absolutely made nothing."