On a tricky day for scoring at a sunny but breezy East Lake Golf Club, Snedeker matched the best score of the week, firing a flawless six-under-par 64 to join Britain's Justin Rose (68) at eight-under 202.
Overnight leader Jim Furyk had also been at eight under with two holes to play but he triple-bogeyed the 17th, after pulling his tee shot into water, on the way to a 72 and a three-way tie for fourth at five under.
Level with Furyk were Masters champion Bubba Watson (70) and Northern Irishman McIlroy (68), who is seeking a fifth PGA Tour victory this year, and his fourth in just five starts.
American Ryan Moore was alone in third at six under after carding a third-round 65 in the PGA Tour's fourth and final playoff event.
"It was probably one of the best rounds I've played this year," a beaming Snedeker told reporters after covering the back nine in three-under 32 on a firm, fast-running layout, a 16-foot birdie putt at the 17th earning him a share of the lead.
"I drove it fantastic, hit a lot of quality iron shots and putted actually okay for me, not great. I left a lot of putts out there.
"To hit 17 of 18 greens in this wind on this golf course, you're striking your ball pretty good. I'm very excited about the standings going into tomorrow."
Snedeker was one of five golfers coming into the elite 30-man event knowing that victory on Sunday would automatically secure him FedExCup playoff honours and the mind-boggling bonus of $10 million.
Also in that quintet was FedExCup points leader McIlroy, who has been the game's most dominant player since cruising to a staggering eight-shot victory in last month's PGA Championship at Kiawah Island.
"It's going to be one of the most exciting Sundays of the year," McIlroy said of the final round at East Lake. "There are a few guys still in it with a chance to win the FedExCup and to win this tournament, so I'm looking forward to it.
"It's great to be in the mix," added the mop-haired Northern Irishman, who mixed four birdies with two bogeys on Saturday. "I'm only three back, so I'm excited about the prospect of a very exciting finish tomorrow."
Tiger Woods, who had battled to a 73 in Friday's second round, carded a 67 to climb into a four-way tie for seventh at four under. Like Snedeker and McIlroy, he would land the FedExCup title by winning the Tour Championship.
"I'm only four back ... it looks like I've got a shot at it tomorrow," the former world number one said after a dogged display that included four birdies and a lone bogey. "Just claw your way back into it."
Asked how tempting it was to be playing for a $10 million bonus in Sunday's final round, Woods replied: "That's not why we play.
"I'm playing for the 'W' (win), and tomorrow I get a great shot at it. All the money, awards and all of that stuff, that comes along with winning championships. If I win the golf tournament, everything's kind of taken care of."
For Rose to land the season-long FedExCup, he knows he is a very long shot who has to win the Tour Championship and hope that players ranked higher in the points standings finish poorly.
"I need a little help from my friends as they say," said the Englishman who was 24th in the standings at the start of the week. "I have the advantage of just playing the Tour Championship.
"The whole year we've been grinding for the FedExCup, and it's still possible. It's not an easy golf course. Things can happen out there. But for me, it's about one tournament."
For Rose to win the playoff title, McIlroy would have to finish no better than 17th in the Tour Championship and Woods no higher than sixth, along with other scenarios