A wild day of rain delays, spectacular charges and a dazzling nine-under 63 from Snedeker at Glen Abbey Golf Course were all overshadowed by Mahan's hurried exit that threw the tournament wide open.
Mahan, a five-time winner on the US Tour, had held a two shot overnight lead but while warming up on the driving range received word that his wife Kandi had gone into labor.
One of the hottest players on US Tour after having played in the final group at both last week's British Open and before that the US Open, Mahan did not hesitate to give up his shot at the $1 million winner's purse, rushing back to Dallas for the birth of his first child.
"On the seventh tee I looked up and I didn't see Hunter's name on the leaderboard and I looked at my caddie, and go, 'What's going on'," Snedeker told reporters. "He goes, 'I think Hunter had to leave because Kandi went into labour.'
"Hunter was going to be hard to catch because he was playing so good.
"With him leaving now, the tournament is wide open and I knew I had a chance if I could keep the momentum going after the first six holes to really ride it out and do something special today."
With Mahan's name off the leaderboard the scramble was on as golfers took advantage of the soft conditions to attack the Jack Nicklaus designed layout.
Snedeker, who began the day a massive eight shots back of Mahan, surged to the top behind an error free display that was just one birdie shy of equaling the Glen Abbey course record of 62.
Lurking one shot back is David Lingmerth, who eagled the par five 18th, to cap off a round of seven-under 65 and put the Swede in contention for a maiden US Tour title on 13-under 203.
Matt Kuchar, the highest ranked player in the field at number six, fired a bogey-free eight-under 64 will start Sunday's final round just two back alongside Jason Bohn, who had a 66.
"I think Hunter right now is playing some of the best golf of anybody in the world," said Kuchar. "To be in the final group of the US Open, final group of the British Open, come here playing just great golf, you knew he was just going to continue to play good golf.
"Not having him in the field kind of bunches the rest of us up and I think kind of gave everybody a chance."
With thunder storms in the area warning horns sounded at exactly noon with spectators and players ordered to evacuate the course.
Before the 80-minute delay several of the early starters, including a trio of former Masters champions Fijian Vijay Singh (2000) and South Africans Charl Schwartzel (2011) and Trevor Immelman (2008) had already mounted charges.
Singh, winner of the 2004 Canadian Open in a playoff over local favourite Mike Weir, got an overcast day off to a bright start going birdie-eagle-birdie to jump up the leaderboard.
The big Fijian would add four more birdies before his round unraveled with a double-bogey at the 15th just prior to the delay followed by a bogey at the 16th right after play resumed but still finished with a 66 to sit just six off the pace.
Immelman also had a 66 but will start five back of the leader while Schwartzel (66) has the most work to do seven shots behind the leader.