Johnson has won three times in the past four years Texas, with his most recent victory overall coming at this event in 2010.
He had a one-shot lead over four players, including one of the sport's current leading attractions.
Jason Dufner, who has won twice and exchanged wedding vows all in the last month, overcame bogeys on two of the easiest holes at the Colonial Country Club to share second place at 65.
Dufner recorded his first PGA Tour triumph at New Orleans in April, got married the next week and won again last Sunday at the Byron Nelson Championship. He has jumped all the way to No. 14 in the world rankings and has virtually wrapped up a spot on this year's U.S. Ryder Cup team.
His recent notoriety earned Dufner an invitation to throw out the first pitch at a Texas Rangers baseball game on Friday.
Joining Dufner at 65 were Tom Gillis, Harris English and Kyle Reifers -- each of whom is seeking his first tour win.
English grabbed a spot in the British Open earlier this week with rounds of 60-63 in a qualifying tournament.
John Daly, making only his fourth start in the United States this year, opened with a 70. Kevin Na, whose difficulty taking the club back received plenty of attention while he was contending for the Players Championship title two weeks ago, returned to action with a revamped and quicker swing. He had three straight birdies in the middle of his round and shot 70.