Walker erupted for 15 fourth-quarter points as the Lakers completed a dramatic 104-101 victory to put them one win away from clinching the best-of-seven Western Conference second-round series.
Walker, who has struggled to establish himself since joining the Lakers on a one-year contract last year after battling a series of injuries, was mobbed by LeBron James and other Lakers teammates after the victory.
James, who scored 27 points in the victory, said he had urged Walker before the game to be ready to help out.
"I've been telling him 'Lonnie you stay ready. We're going to need you at some point'," James said.
"For him to be as young as he is, to have that professionalism, to come out and stay ready when his number was called -- we don't win this game without Lonnie Walker tonight, that's for sure."
Walker said he had been primed to make a contribution.
"It means the world to me, truly," Walker said.
"By the grace of god, I was given an opportunity to play. Night-in, night-out I've been going to the gym, everyone's been telling me to stay ready -- Bron, coach, all of them. So when the call came I was locked in the entire time."
The Lakers will take a 3-1 series lead to San Francisco for Game Five on Wednesday with the Warriors needing to win to keep the series alive.
For long periods of Monday's game, it looked as if the champions were poised to square the series.
Stephen Curry finished with 31 points, 10 rebounds and 14 assists in a dazzling playmaking performance that helped Golden State take an 84-77 lead into the fourth quarter.
But the introduction of Walker changed the game, with the unlikely hero scoring 15 points as the Lakers outscored their opponents 27-17 in the final frame to clinch victory.
'A RESILIENT BUNCH'
"We're just a resilient bunch," James said. "They hit us with haymaker after haymaker after haymaker, trying to get us to fall.
"We just stayed in the fight. It's a 48-minute game, a 12-round boxing match. And we stayed in it for 12 rounds and were able to pull through."
Warriors coach Steve Kerr paid tribute to Walker's fourth-quarter performance.
"I don't know whether he scored all their points in the fourth quarter but it felt like it," Kerr said. "He made a huge impact.
"A lot of times that's what the playoffs are about. A close game and somebody comes in who you don't expect to make an impact. The fourth was about Walker."
In Monday's other playoff game, Jimmy Butler scored 27 points as the Miami Heat outmuscled the New York Knicks to take a commanding 3-1 lead in their Eastern Conference playoff series.
Butler was one of five Miami players to finish in double digits as the Heat edged to a 109-101 victory that leaves the eighth seeds just one win away from a place in the Eastern Conference finals.
It was another ruthlessly clinical performance from Miami, who looked hungrier and sharper throughout at both ends of the court.
Miami hauled down 13 offensive rebounds to New York's eight and out-hustled New York defensively with 31 defensive rebounds to the Knicks' 27.
Miami coach Erik Spoelstra said his team was now reaping dividends from their resilience forged in the regular season, when they were forced to battle through the play-in tournament to reach the playoffs.
"We found the value in the grind of the regular season, the grind and the struggle," Spoelstra said.
"Without that struggle -- where we had to find different solutions to win, with different guys stepping up for these kind of moments -- if we didn't do that in the regular season you have zero chance to be able to do that in the playoffs."
The Knicks' shooting from three-point range was also found lacking, with New York making just 9-of-28 from beyond the arc in contrast to Miami's 13-of-39.
Butler finished with 27 points, 10 assists and six rebounds while Bam Adebayo produced another towering defensive display with 23 points and 13 rebounds.
Jalen Brunson led New York's scorers with 32 points, while RJ Barrett added 24 and Julius Randle 20.
The Knicks now must win all three of their remaining games to stay alive in the playoffs, while Miami can book their place in the conference finals with a win in Game Five in New York on Wednesday.
