Mahomes singled in the winning run in the 13th inning and protected the last of three late leads for the Mets, as New York beat the Chicago Cubs 5-4 Sunday.
The victory helped the Mets keep pace with first-place Atlanta in the NL East. The Braves, who lead by a half-game, beat Philadelphia 12-4 Sunday.
"I've been pitching for 12 years in the big leagues on and off and I've been stuck on 11 strikeouts a few times," Leiter said. "I knew with my strikeouts and a few walks that my pitch count would be up. It's hard to complete games like that."
Leiter, who threw 135 pitches, was unable to pitch the first complete game of the year for New York. The Mets have not thrown a complete game since last September, a span of 127 games 13 short of Anaheim's major league record.
Mets manager Bobby Valentine used all of his available players except Luis Lopez, including newly acquired players Hamilton, Chuck McElroy and Billy Taylor.
"They all did really well," said Valentine who had hoped to break Taylor in in a non-save situation. "(Taylor) got here in the fifth and I said, `We'll pitch you to a couple batters, not in a save situation, before the ninth. Don't worry about the signs, just go out there."'
The bullpen nearly blew the game for the Mets. Henry Rodriguez hit Benitezs first pitch of the ninth into the center-field bleachers for his 19th homer, tying it at 3.
After Edgardo Alfonzo hit a sacrifice fly to give New York a 4-3 lead in the 10th, Benitez walked the first two batters in the bottom half. One out and two pitchers later, John Olerud booted Rodriguez's bases-loaded grounder and Manny Alexander scored to tie it at 4.
Dennis Cook then got pinch-hitter Jose Nieves to ground into an inning-ending double play.
Two outs later, Sanders intentionally walked Benny Agbayani to face Mahomes (5-0), who blooped a single to shallow left, scoring Cedeno.
"I always hit well in high school," said Mahomes, who batted .583 his senior year. "I wanted to hit. I had a chance to win the game. I heard Bobby (Valentine) say, `I think I'm going to let him hit.' That gave me more confidence."
Mahomes, 4-for-10 with three RBIs in his first season in the NL, got Sammy Sosa on a groundout to end the 12th. Mahomes allowed a two-out double to Sanders in the 13th but struck out Jeff Reed to end it.
The Cubs fanned 20 times, a season high.
Leiter, who threw 136 pitches, bettered his previous strikeout high of 11, which he had accomplished four times, most recently July 28, 1997, when he pitched for Florida against Cincinnati.
The Mets have not thrown a complete game since last September, a span of 127 games 13 short of Anaheim's major league record.
Mark Grace went 3-for-5, including an RBI single, to move two hits shy of 2,000 as the Cubs lost for the seventh time in their last nine games.
"I would've liked to get it today. I had enough at-bats," Grace said.
Sosa, tied with Mark McGwire for the major league lead with 40 homers, went 0-for-6.
The Cubs took a 2-0 lead in the fifth on RBI singles by Alexander and Mark Grace.
"I hate to be traded somewhere and have the team lose the first day," Hamilton said. "The deal last year (when he was traded from San Francisco to Colorado) hurt my ego quite a bit but coming here where we're a half-game out makes me feel 100 percent better. The Mets are a contender and got me for a reason."
Cubs rookie Kyle Farnsworth gave up two hits over five innings in his first start since July 3. Farnsworth was subbing for Terry Mulholland, who was traded Saturday to Atlanta along with infielder Jose Hernandez.
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