Walker's third-inning homer off Jon Moscot (0-2) extended the Mets' lead to 3-1 and tied the team record for home runs in April.
Harvey (2-3) won his second consecutive start after losing his first three. He overcame a first-inning home run by Zack Cosart and gave up two runs and seven hits in six innings. Mixing a changeup with a fastball that reached 97 mph, Harvey struck out a season-high seven and walked one.
Addison Reed pitched a perfect ninth for his first save. Coming off a weekend sweep at Atlanta, New York has won nine of 10 to close within 1 1/2 games of NL East-leading Washington.
