He was attracted by the punishing defense, the strong running game and a coaching staff he trusted.
The 49ers announced Thursday that Manningham had signed a two-year deal to join the team he helped knock out in the NFC championship game two months ago in their latest offensive upgrade at wide receiver.
"I knew (playing with Manning) was a possibility but I wasn't picking them because of that. I know Peyton Manning is a great quarterback and everything but if you don't have a good defense ... offense can be potent but there's really no point."
San Francisco has now made moves to improve the offense, especially at wide receiver, where Manningham and fellow newcomer Randy Moss will join Michael Crabtree in a unit that should help Smith develop even more as a potent passer. Crabtree was the only 49ers wideout to catch more than 20 passes last season.
"I feel like I can come there and make an impact and be a great addition to this offense," Manningham said.
Manningham has 160 catches for 2,315 yards and 18 touchdowns in his four-year career after being drafted out of Michigan in the third round in 2008. He was limited to 12 games last year because of injuries and had only 39 catches for 523 yards and four scores in his least productive season since his rookie year.
"We are very pleased to add a player like Mario to our team," general manager Trent Baalke said. "Although it is still early in his NFL career, he is a productive wide receiver with a lot of big game experience. He will be a nice complement to our current wide receiver group and a good fit for our offensive system."
Manningham took off in the postseason, catching TD passes in wins over Atlanta and Green Bay to get New York to the NFC title game. He then had a go-ahead 17-yard touchdown catch in the fourth quarter of the Giants' overtime win over San Francisco to get to the Super Bowl and capped the postseason with a memorable catch.
His over-the-shoulder 38-yard catch between two defenders, on which he managed to stay inbounds and hold onto the ball despite being hit, started the game-winning, 88-yard touchdown drive for the Giants.
But despite that, New York showed little interest in keeping Manningham, who ultimately picked San Francisco over St. Louis.
"That's the business part about this system, about football. I would have wanted to come back to the team that I started with. I won a Super Bowl with those guys," he said. "This is my new team now."
Having played the Niners in both the regular season and playoffs this past season, Manningham knows how good San Francisco is on defense.
"They swarm, they'll hit you and they'll cover you," Manningham said. "I feel like they have the best linebacking corps in the league. They have athletic defensive backs, d-line, they're all really good. It's a great defense out there."