Maligned throughout his three-year stint with New York, Collins completed 30 for 47 passes for 338 yards, but he went more than 59 minutes without a touchdown and fumbled in the end zone.
It was the 18th fumble of the season for Collins, who is just three off an NFL record set by Tony Banks with the St. Louis Rams in 1996.
But Collins shook off another shaky day on the decisive drive, going seven of 10 for all 96 yards and finding Hilliard over the middle for the winning score.
A byproduct of the result was the clinching of a playoff berth by the New England Patriots. New England, which took the lead in the AFC East Saturday with a win over the Miami Dolphins, needed either Baltimore, the New York Jets or Seattle to lose Sunday to clinch a post-season spot.
New York moved within two games of NFC East-leading Philadelphia and the Giants play the Eagles on the road next week.
"You see your season melt before your eyes and you have to be composed," said New York coach Jim Fassel.
Next week's game would have been meaningless if not for Collins, who completed seven of 10 passes for all 96 yards on the decisive drive.
"It's about just keep fighting," said Collins, who was 30 of 47 for 338 yards. "You go through a game, you have ups and downs. You have to keep it together for 60 minutes."
The final drive was especially surprising considering the Giants began their previous possession at their own one and went three-and-out. But Collins remained confident on the bench.
"I said, `We're going 99 yards," Collins yelled in the locker room. "We came off and got it again. `OK, we're going 96 yards.'"
On first and 10 from the Seahawks' 25, Collins looked downfield for Hilliard and watched Reggie Tongue drop an interception. Two plays later, linebacker Anthony Simmons got his fingertips on a pass intended for Jurevicius.
"I had a chance," Simmons said. "I was a half step off. If I had a little more jump on the ball, I would have had it."
New York still was left with third and goal from the seven before Collins went to Hilliard, who found a seam between cornerback Willie Williams and safety Marcus Robertson.
"They had combo coverage on Amani and Joe," Collins said. "That left Ike free to do what he can to get open. I saw (Williams) was playing him to where he couldn't get across his face.
"I knew Ike would plant his foot and go. It was simultaneous. It's not the first time. That kind of chemistry is hard to find."
Collins had chemistry with Hilliard and Toomer all day. The two combined for 15 catches totaling 229 yards after looking non-existent before the final drive in last Saturday's 17-13 win over Arizona.
"We had to step up," Hilliard said. "We were ripped all week in the meeting room. Last week was our worst game. Coach reminded us of that every day."
The Seahawks will be reminded of their next-to-last possession, which began at the Giants' 37 and included three plays totaling three yards.
"My mindest was to at least get in field-goal range, don't take a sack," linebacker Chad Brown said. "We had an opportunity for the offense to end the game, but we didn't take advantage."
The Seahawks (7-7) did not take advantage of their chance to stay in the thick of the wild card race. They need to win their last two games and get a lot of help.
"This hurts the most out of all the games we lost," said Brown, whose team dropped its third straight on the road. "We could have won and we let it slip away."
The defending NFC champion Giants won their second straight following a two-game skid. But the victory will not mean much unless they can defeat the Eagles next Sunday.
"We lose, and again, we're done," Fassel said. "We win, we keep going. It's like a playoff for us. We have to treat it that way."
The game was tight throughout as the teams traded long touchdown runs, defensive touchdowns and field goals in the first half for a 17-17 tie.
Morten Andersen made it 24-20 early in the fourth with a 33-yard field goal.
Alexander also rushed for 96 yards and a score for Seattle, which had 259 yards of total offense to 420 for New York. Hasselbeck finished 15 of 26 for 185 yards.
Both Hasselbeck and Collins had key fumbles in the second quarter.
Defensive end Michael Strahan returned Hasselbeck's fumble 13 yards for a score to give the Giants a 14-10 lead 5:42 before halftime. But just 2:28 later, defensive tackle John Randle fell on a fumble by Collins in the end zone.