Mason, who re-signed with the Bucks for two years this offseason, made a 14-foot jumper with 13.6 seconds left off Williams' 12th assist, and Memphis' Pau Gasol missed a tying 3-pointer, giving Milwaukee a 102-99 victory over the Grizzlies on Wednesday night.
"One dribble, pull up, I shoot tons of them in practice," Mason said. "I just took to the elbow and pulled up with confidence."
Mason, who was traded from Seattle to the Bucks in 2003 as part of the Ray Allen deal and then dealt to New Orleans two seasons ago, deflected the credit back to Williams.
"It was the point guard running the show," Mason said. "He got it done."
It almost was all for naught because of Gasol's heave at the buzzer.
"I got a pretty good look and it wasn't contested," Gasol said. "I just caught it and turned around and shot it."
Instead, it clanged harmlessly off the rim.
After Mason's basket, Williams stripped Kyle Lowry, who wanted a foul. Instead, Williams went to the line to hit two shots of his own to give the Bucks a 102-98 lead.
But the Grizzlies refused to go away, nearly tying it when Mike Miller missed the second of two free throws with 2.5 seconds to play. Gasol, who had 16 points, missed the 3 from the right corner as the Grizzlies finished 6-of-18 from beyond the arc.
"We just aren't a good 3-point shooting team," Grizzlies first-year coach Marc Iavaroni said.
Memphis, fresh off a big victory against Houston at home, looked to have its first two-game winning streak this year after it took 73 games for one last season.
Trailing by 11 in the fourth quarter, the Grizzlies drew even when Rudy Gay, who had 26 points, hit a 3-pointer to tie it at 98 with 34 seconds left. That's when Williams set the screen for Mason, who hit his jumper over Darko Milicic to give the Bucks a 100-98 lead and set up the final sequence.
Miller finished with 17 points for Memphis, while Williams had 10 points and Mason added 15 for the Bucks.
Michael Redd scored 31 points, and Andrew Bogut had 23 points and 12 rebounds to help Milwaukee overcome a bad early shooting effort and win its third straight home game to go with four losses, all on the road.
Milwaukee was 34.6 percent from the field in the first half, capped by a 4-of-13 effort by Redd.
"It wasn't like they were bad shots," Bucks coach Larry Krystkowiak said. "We missed a number of open shots. At some point, we're going to kind of come out of the funk."
Gasol, recently criticized by Iavaroni for taking plays off, got a tongue lashing just before the half for arguing a no-call with officials instead of running back down to play defense.
Gasol responded early in the third quarter and the rest of the Grizzlies followed after a pair of easy baskets, the second off a pretty feed by Milicic, who finished with 10 points and 13 rebounds.
A few moments later Milicic got free for a dunk, and Gasol fed Damon Stoudamire for a 20-footer. Gay added a jumper and Stoudamire's steal and layup capped an 8-0 run that gave Memphis its largest lead 57-53 midway through the quarter.
That's when Redd caught fire.
He hit his first five shots of the second half and went 7-of-10 from the field.
Redd added a 3-pointer to push the Bucks' lead to 78-71 going into the fourth, which they stretched to double-digits before the Grizzlies came roaring back.
"Defense is going to help win games, but you need to put the ball in the basket," Redd said. "It's always good when you make shots."
Bogut, who had already faced a bevy of NBA stars this season including Dwight Howard, Ben Wallace, Chris Bosh, Yao Ming and Tim Duncan, outplayed Gasol, and rookie Yi Jianlian had six points and 13 rebounds to help his frontcourt mate.
"He did a great job defensively," Iavaroni said of Bogut. "Last year, I thought that was definitely a weak point in his game. He stood in there pretty tough tonight and made Pau work."
Charlie Villanueva scored 13 points for the Bucks, all in the first half. ... The Bucks activated G Awvee Storey after F Michael Ruffin broke his left wrist Sunday against the Spurs. Ruffin is expected to miss four to six weeks. ... Neither Krystkowiak nor Iavaroni thought Yi and Gasol were similar. Yi had been compared to Gasol in the days leading up to the draft. ... Grizzlies F Brian Cardinal returned after missing the previous game with a bruised right knee.