Keselowski, who had to finish only 21st or better to wrap up the driver's championship with two races left, crossed the line behind winner Carl Edwards and runner-up Kyle Busch to earn his 24th top-five result of the season.
"Part of you feels honored to do it for Roger," Keselowski said. "To get to see him carry that (NASCAR) trophy and do something he's never done, it's hard to give a billionaire something. It's pretty cool."
While Keselowski did celebratory doughnuts in his No. 22 Dodge, Edwards got the checkered flag from his 28th career Nationwide victory and went into the stands to share a moment with fans sitting on the frontstretch.
Edwards charged ahead on the final restart, a green-white-checker finish that added five laps to the scheduled 200-lap race. Busch was denied a NASCAR-record sixth consecutive win at Texas and claimed Edwards took off early.
"Carl Edwards jumped the restart by about three (car) lengths," an obviously annoyed Busch said. "The race is over. Guy's in Victory Lane. Doesn't matter."
Keselowski has held the points lead since winning at Talladega in the eighth race this season April 25. He got to Texas with a 485-point lead over Edwards that got trimmed by only 20 Saturday.
"Going through the last few weeks knowing that it looked pretty good to win the championship, I couldn't help but think of all the things that could go wrong," Keselowski said. "When you think of those things, I don't want to get my hopes up until it's really there. It's here. It really happened."
Busch led three times for a race-high 107 laps. Edwards also led three times for 59 laps and won by 0.702 seconds with an average speed of 142.270 mph.
While Keselowski has clinched the driver's title, the No. 22 is still second in owner points, 81 behind the Joe Gibbs-owned No. 18 driven by Busch.
"What a terrific season it has been for Brad," said Penske, who has won 12 open-wheel titles and a record 15 Indianapolis 500s.
Edwards actually withstood two late restarts to beat Busch. The first was after his 1.69-second lead was wiped out when a car four laps down spun, bringing out the caution.
On the restart on lap 191, Busch went down inside while Edwards went high around Trevor Bayne, who was leading only because he didn't pit that final stop. Once they got around Bayne, Edwards and Busch were going side-by-side before Edwards pushed ahead.
A few laps later, Brian Scott hit the wall hard and parts were coming off his car. But there wasn't a caution until two laps later when Clint Bowyer's car starting smoking on the frontstretch.
Edwards had the inside spot on the final restart, quickly charging ahead and was never really challenged over the final two laps to the checkered flag.
"I don't expect him to be happy," Edwards said when asked about Busch's early jump accusation. "I just did what I had to do to win the race. We won the race. It just really doesn't matter what he says."
Winning a championship is a big step for Keselowski, a talented young driver best known for some high-profile run-ins with Edwards. Keselowski first gained national attention in April 2009 at Talladega after bumping Edwards on the final stretch, sending his rival's car flying into the catch fence in a frightening crash.
After Edwards finished celebrating his victory in the stands Saturday, he leaned into the cockpit to congratulate Keselowski before getting back into his own car.
"It's his first championship. They've done a good job," Edwards said. "We've had our differences and our battles, but today is his championship day and he beat us."