Playing on Mother's Day, just three days after his mother's sudden death, the right winger scored 3:34 into the game and the Rangers fed off of his raw emotion to tie the Eastern Conference semi-finals series at 3-3.
With their second straight do-or-die win, New York head into Tuesday's decider in Pittsburgh with all the momentum.
St Louis's goal lifted the Madison Square Garden crowd, who chanted his name as the player unleashed a big fist pump and pointed in the direction of his father and sister, both in attendance for the contest.
"It's been a tough day for everyone. To have (my family) here going through this tough time - I couldn't be happier," St. Louis told reporters.
"I know my mom would be proud."
Cari Hagelin added a second goal three minutes after the opener and although Brandon Sutter pulled one back for the Penguins in the first period, Derick Brassard sealed the contest for the Rangers late in the second.
New York goaltender Henrik Lundqvist made 36 saves in the victory.
"The first period hurt us," said Pittsburgh captain Sidney Crosby. "We fought hard and tried to get back, but you can't continue to do that in the playoffs."
The Rangers held Pittsburgh forwards Crosby and Evgeni Malkin to a combined three shots on goal as New York adopted a physical approach to deal with the Penguins captain, putting a body on him nearly every time he touched the puck.
After appearing to have control of the series, Pittsburgh have been out-scored 8-2 over the last two games to fall one loss from elimination.
New York, on the other hand, are playing inspired hockey as they rally around their grieving team mate.
"It's probably one of the cooler things I've been a part of my in my professional career," Rangers center Derek Stepan said.
"With what he's been through in the past couple of games, for him to score a goal and the way he did, it is something I won't forget for sure."
(Writing by Jahmal Corner; Editing by John O'Brien)