Staked to a 7-1 lead by the ninth, Reds starter Mike Leake appeared to be cruising toward his first career complete game.
Then it all unraveled.
Greg Dobbs hit a three-run homer and pinch-hitter Cody Ransom followed with a tying, two-run shot off Francisco Cordero to make it 7-all.
Raul Ibanez opened the 10th with a double off Arthur Rhodes (3-3) and Howard went the opposite way for his 17th homer of the season.
Ryan Madson (2-0) tossed a scoreless inning for the win.
Brian Schneider hit a game-ending homer in the 12th inning to give the injury-depleted Phillies a win over the Reds on Thursday.
The Phillies couldn't solve Leake until the ninth.
Howard's RBI single made it 7-2. After a single, Dobbs followed with a three-run homer off the right-field foul pole that sent the 82nd straight sellout crowd at Citizens Bank Park into a frenzy.
It was only the start of a wild ending before the postgame fireworks.
Leake was finished after the longest outing of his career. Cordero retired the first batter he faced, then walked pinch-hitter Ben Francisco. Ransom followed with a drive to right-center that made it 7-all.
Ransom's homer was his first of the season and his second career as a pinch hitter.
Jonny Gomes hit a three-run homer to help the Reds roll early.
Gomes drove in four runs and Miguel Cairo had three hits. They delivered on some late, pregame shuffling of the lineup once Scott Rolen was scratched with a sore back. Cairo took his spot at third base, Gomes was bumped from fifth to fourth in the lineup and it all helped the NL West-leading Reds in a big way.
Leake was stingy with his pitches, throwing only 45 through the first five innings. He also had three hits.
The Phillies committed two errors, had a baserunner picked off first and another thrown out trying to stretch a single into a double by the fifth inning.
The Reds scored three in the first inning and another in the second off starter Joe Blanton.
Blanton, who agreed to a $24 million, three-year deal in the offseason, heard a smattering of boos when he left after 5 1-3 innings.
Philadelphia shortstop Jimmy Rollins booted a grounder for an error in the first and All-Star Joey Votto singled on a perfect hit-and-run. Gomes hit his 11th homer for a 3-0 lead.
In the second, Gomes ripped a liner off Dobbs' glove at third for a run-scoring single.
Leake, who never pitched in the minors, was sensational for 8 1-3 innings and didn't tire in the muggy conditions while running the bases. His RBI single in the ninth made it 7-1. He became the first Reds pitcher to have three hits in a game since Bobby Livingston had four in 2007.
Maybe the scouting report was "swing early" because the Phillies didn't make Leake work to throw pitches. He got a visit from pitching coach Bryan Price in the ninth after allowing a run, then was yanked after Dobbs went deep.
NOTES: Reds SS Orlando Cabrera was 0 for 6. ... Phillies manager Charlie Manuel expects C Carlos Ruiz (concussion) to return to the team Saturday. ... Manuel is open to a trade if it will help the team. "There are things we can do," he said.
