The Eastern Conference-leading Red Bulls (15-9-7) have 52 points, while the Sounders (15-8-6) picked up their 51st point to climb back into a tie with Real Salt Lake for first place in the Western Conference.
After New York saw two earlier chances at the tying goal bounce away - one off the crossbar in the 61st minute, one off the left post in the 63rd - Cahill got his chance at the equalizer when the Sounders were unable to clear the ball out of their penalty area. He got a foot under it, and from 10 yards away toward the left side, beat Seattle goalkeeper Michael Gspurning to the far corner.
"I thought about dragging it with my right foot and smashing it with my left," Cahill said. "The keeper was sort of just to his right of it, and I tried to find the corner, and it hit."
Cahill's ninth goal of the year, scored in front of 39,083 fans and in the midst of a driving rainstorm, tied him for the team lead with Thierry Henry, who missed the game with an Achilles injury.
"You play for moments like this - defining moments that change the game," said Cahill, in his second year with the Red Bulls after playing previously with Everton of the English Premier League. "My whole career, I've looked to try to make a difference in front of big crowds and (against) the best teams in leagues everywhere I've played. It was unfortunate not to get the three points, but it's a pleasure to play here, and I'm proud of what we achieved tonight."
Brad Evans gave the Sounders a 1-0 lead with a penalty kick midway through the 45th minute.
Playing inside the box on the right, about 6 yards up from the goal line, Evans' attempted short cross deflected off the left hand of Red Bulls defender Markus Holgersson. Referee Kevin Stott immediately pointed to the spot, and Evans drilled his kick low into the back right corner past goalkeeper Luis Robles, who dove in the same direction, but not in time.
Evans' goal was his fourth of the season, two of them on PKs.
"I always feel comfortable stepping up there," said Evans, who has seen duty with the U.S. national team during its World Cup qualifying matches. "I've just got to keep it going, and pick my spot, and get it there."
The Sounders had the run of play during the first half, with four near-misses, including an Obafemi Martins header that went barely to the right of a wide-open net.
"Obviously, if you put that one in, it changes the complexion of the game," Sounders coach Sigi Schmid said. "I thought we had some good looks and good chances. Robles came up with a couple good saves, but I didn't think it was our best soccer. We got a point, and we still have some games in hand. But when we play at home, we'd like to win."
Red Bulls coach Mike Petke said the rain, which was steady during the second half, was actually helpful.
"I love this weather," he said with a laugh. "It was possibly the best thing that could have happened to us. Although they are used to it here, any time you can make the environmental conditions a bit uglier in an environment like this, we'll take it."