The 33-year-old former England captain smacked the screamer in the 10th minute of the 1-0 victory Wednesday at Audi Field in Washington, D.C.
"I thought it was nice and it was special to score the goal this evening," Rooney told reporters. "It's something that I've practiced so many times. Different managers and teammates are like why are you practicing this, you won't score from this position, but for nights like tonight that's why I do practice it. It was a good strike and an important goal for us."
United cleared the ball out of its own box at the start of the sequence. The ball initially fell out to City, but was deflected ahead to Rooney. The veteran forward watched the ball roll toward midfield, before looking up and spotting City goalie Brian Rowe off of his line.
Rooney used his next touch to blast a long-range chip toward the goal. The ball carried through the center of the City box, going over Rowe's head before bouncing into the net.
Rooney now has 11 goals after scoring in three consecutive matches. United has not lost in its last five MLS matches at Audi Field.
"It has been fantastic," Rooney said. "I've enjoyed my time here over the course of the year and there is still a lot more to come. We know we can play better."
United hosts Toronto FC at 8 p.m. Saturday in Washington, D.C.