The Flyers, 9-0 in Games 4 through 7 during this year's playoffs, built a 3-1 first-period lead on goals by Mike Richards (power play), Matt Carle and Claude Giroux.
A third-period goal by Leino - it deflected off the back of the Hawks' Kris Versteeg - made it 4-1 with 13 minutes, 17 seconds left and proved to be the game-winner because of a furious Chicago rally.
It gave Leino 16 playoff points, one more than the rookie record set by Brian Propp in the 1987 playoffs. Leino had missed about 10 first-period minutes after a vicious open-ice hit by Brian Campbell.
The Flyers have won 10 straight home games against Chicago since 1996. It was their second straight win in the series, and they handed Chicago back-to-back road losses for the first time since Nov. 27 (at Anaheim) and Nov. 28 (at Los Angeles).
But it was far from easy.
A 5-on-3 power-play goal by Dave Bolland cut the lead to 4-2 with 7:59 left, but Michael Leighton made two key stops on the remaining 5-on-4 advantage to protect the two-goal lead.
Chicago got to within 4-3 on Campbell's goal with 4:10 to go.
And then the Hawks applied pressure.
And more pressure.
Leighton stopped Brent Seabrook's redirect with 2:15 left, and the Flyers got sloppy in their own zone and couldn't clear the puck for long stretches.
The sellout crowd couldn't exhale until Jeff Carter's empty-net goal iced it with 26 seconds left.
The Hawks, aiming for their first Cup since 1961, had been 4-0 this year in playoff games following losses and had outscored their opponents, 14-3. But the Flyers took control early and would not allow Chicago to get into a rhythm - until the latter stages of the game.
In their history, the Flyers are 13-6 in series that are tied at two games each; the Hawks are 13-18 in series that start 2-2. (Chicago was 14-0 is series when the won three of the first four games.)
Both coaches have been playing mind games with their opponents, trying to get a psychological edge.
Flyers coach Peter Laviolette has taken verbal jabs at Chicago goalie Antti Niemi, saying that it's difficult for a rookie to excel with the weight of the Blackhawks' championship drought - 49 years - on his shoulders.
Laviolette may have gotten into Niemi's head. The 26-year-old goalie stole Game 2 with a brilliant third period, but he hasn't been nearly as sharp since Laviolotte went on the offensive before Game 3.
Niemi allowed three goals on just eight shots in Friday's first period as the Flyers built a 3-1 lead.
Then again, defenseman Niklas Hjalmarsson didn't help Niemi's cause as his two turnovers contributed directly to the Flyers' first two goals.
With the Flyers on the power play, Richards stripped the puck from the defenseman near the side of the net and scored on a spin-around shot after just 4:35.
"It's nice to help the team, especially on the power play," said Richards after his first goal of the Finals. "It was a face-off. He hung on to the puck a little bit too long. I just grabbed it from him and threw it at the net and it went in."
About 10 minutes later, a ferocious forecheck by James van Riemsdyk, Arron Asham, and Giroux enabled the Flyers to keep the puck in Chicago's end, and when Hjalmarsson's clearing attempt ended on the stick of Matt Carle, the defenseman fired a shot from the high slot past Niemi to make it 2-0.
Leighton played solidly in the first period, but the Hawks got to within 2-1 on a goal by Patrick Sharp with 1:28 left in the session.
Scott Hartnell's clearing pass went to Sharp, whose long shot appeared to deflect off the stick of defenseman Braydon Coburn and past Leighton.
It took the Flyers 51 seconds to answer.
Kimmo Timonen wound up for a shot, drawing Niemi a few feet out of the net. But Giroux snuck around from behind the net - Niemi apparently didn't see him - and Timonen put a pass right on the center's stick.
Giroux, on the doorstep to the left of the goal, had a wide-open net as Niemi couldn't get back in time. He rammed in the goal, putting the Flyers ahead, 3-1, with 37 seconds left in the first period.
"It was a great play by Kimmo; he has great vision," said Giroux, whose overtime goal gave the Flyers a 4-3 win in Game 3. "It was huge to get a goal back after they scored late in the period."
After Friday's morning skate, Laviolette was asked about the Flyers trying to rattle Niemi with traffic in front.
Laviolette took the bait.
"Well, I said before that we feel there's some weaknesses we can exploit there," he said. He later added some more words for the Blackhawks, just in case they were listening.
"We're in a position where the pressure for us, it's almost like we're on borrowed time," he said. "The pressure, I think, is more for teams that are expected to win, as the Blackhawks are. Everybody picked them before the series."
Chicago coach Joel Quenneville (surprisingly) hasn't mentioned Leighton's post-season inexperience, but he has talked about Chris Pronger - and intimated how he beilieves the veteran defenseman gets away with penalties.
Quenneville has been trying to get Pronger, a 35-year-old veteran playing in his third Finals, off his game.
It hasn't worked.
As for Niemi, after a shaky start, he made several quality saves in the second period, including stops on Carter, Richards, and Hartnell in the final four-plus minutes of the period to keep the Hawks within 3-1.
Before Friday, Niemi had stopped 109 of 112 shots (.973 save percentage) in the four post-season games following a loss this year.
Leighton, meanwhile continued his steady play, and the first two periods were his best of the Finals. He made 23 saves in the first two periods
In the first 40 minutes, the Flyers were outshot, 24-18.
"We've been forced to play a little more defense tonight," Danny Briere said after the second intermission, adding that the Flyers' forecheck wasn't at its best. "We're taking a lot of shot lanes away, which is good. We're keeping them to the outside, for the most part."
And moving closer to their first NHL championship in 35 years.
Contact staff writer Sam Carchidi at 215-854-5181 or firstname.lastname@example.org. Follow on Twitter at www.twitter.com/BroadStBull