Kevin Weekes, making his second consecutive start for the first time since early November, stopped 15 shots as his teammates played tenacious defense and allowed their fewest shots this season.
"That's where we have improved the most," said Cullen, who scored his first goal in 18 games. "We've really buckled down defensively and eliminated other team's chances for the most part.
"We've spent a lot of time working on the defensive side of the game. I guess one of the main things is back pressure from our forwards which is allowing our defensemen to keep a tight gap and not give the offensive players much space. We've really improved in that area, probably more so than any other area."
Cullen said it was important for the Rangers to stay focused while facing the worst team in the Eastern Conference.
"It's one of those games where you really have to try not to think about what's going on in that locker room," Cullen said. "We really just have to focus on our game and what we've been doing and what we need to do.
"I think we did a pretty good job. That's our leadership. Shanny (Brendan Shanahan) and Jags (Jaromir Jagr) are helping to steer the ship."
New York coach Tom Renney wasn't impressed with the Rangers' effort on the back end.
"Defensively we were OK," Renney said. "But we were just sloppy enough where it was scary enough. We gave them opportunities to still direct pucks to the net and gave them an opportunity or two on the power play that still could have meant the difference in the game the other way."
Michael Nylander also scored for the Rangers.
R.J. Umberger scored for the Flyers, who dropped their fourth straight game.
"It's another loss," Umberger said. "I don't know if there is anything to really feel good about. We obviously got outshot and outchanced pretty bad. But we still found a way to be in the game toward the end and had a chance to win it."
Immonen broke a 1-1 tie at 15:55 of the second period with a backhander from the right circle that seemed to handcuff Flyers goalie Antero Niittymaki, who allowed the puck to trickle between his pads.
Immonen said it was a lucky shot.
"I didn't know what to do," he said. "I just threw it in. I need to be lucky."
It was Immonen's first goal in eight games, since last April 8 at Boston. He has played in four games with the Rangers since being recalled from Hartford of the AHL.
Nylander sealed the victory with 4:27 left, scoring off a give-and-go with Martin Straka, who extended his career-best point streak to 11 games.
Cullen put the Rangers ahead 1-0 at 6:56 of the first period. After a wraparound pass from Shanahan hit his skate, Cullen punched in a rebound of his initial shot.
The Flyers tied it 25 seconds later when Umberger took a centering pass from Todd Fedoruk and put a one-timer over Weekes' outstretched glove.
Niittymaki, who started his 10th straight game, made 31 saves. He was particularly strong in the first period when he stopped 15 of 16 shots, including a spectacular save with his extended right foot on a point-blank shot by Straka.
"We did some really good things," Flyers coach John Stevens said. "But we just had trouble sustaining anything."
- The Flyers have an NHL-low three wins at home.
- The Rangers have won five of six in Philadelphia.
- Straka and Philadelphia's Randy Robitaille hit goal posts with first-period shots.
- The Flyers matched their lowest shot total of the season.