Howard hit his 28th homer of the season and the NL East-leading Phillies tied a season high with 17 hits.
Hamels (10-10) tossed four-hit ball over seven innings and extended his consecutive scoreless innings streak to 25. He struck out five and won his third straight start.
His record would be well over .500 if not for receiving the fourth-lowest run support (3.18 runs per start) of any starter this season entering the game.
He got plenty in Philadelphia's third straight win. Phillies shortstop Jimmy Rollins left with tightness in his right hamstring. He is day-to-day.
Gaby Sanchez hit a three-run homer for the Marlins as part of a six-run eighth off Nate Robertson.
Hamels was sensational and earned a standing ovation when he struck out Brad Davis to end the seventh. Hamels, the 2008 World Series MVP, might have 15 wins by now if the Phillies had some pop in their bats in his starts.
The Phillies were shut out in four of his starts - including consecutive 1-0 losses in early August. The Phillies entered having scored only 19 runs in Hamels' last 11 starts. They scored two or fewer runs in 13 of his first 28 starts.
They tried to make it up to him from the start Wednesday, scoring runs in each of the first five innings and staked him to a 10-0 lead.
Howard did the bulk of the damage against Andrew Miller (1-1). He had a two-run single in the first inning and went deep with an opposite-field, three-run shot to left in the fourth that made it 7-0.
Miller was long gone when Howard's line-drive single to right in the seventh put the Phillies ahead 10-0. Howard was batting .188 in 16 games since returning from the disabled list on Aug. 21.
Put in perspective, the Phillies scored 10 runs combined in Hamels' last six starts.
Hamels never really groused about his lack of support and kept giving the Phillies the kind of quality starts that has them convinced they can make another deep October run. The Phillies started the day a half-game ahead of Atlanta in the NL East as they make a run toward their fourth straight division title.
The Phillies don't know if they will have to move on without Rollins.
Rollins - who has slumped so badly this season he was dropped from leadoff to sixth in the batting order - gingerly ran to second base and didn't slide on a two-out double in the third. He didn't have his usual burst around third base when he scored on Carlos Ruiz's single.
Rollins slid at home and slowly walked back to the dugout. He had his head down as he talked with manager Charlie Manuel. Rollins was 2 for 2 and was replaced by Wilson Valdez.
The three-time All-Star shortstop has twice been on the 15-day disabled list this season because of a calf problem. He got hurt warming up before the Phillies' home opener and missed 31 games, then re-injured himself less than a week later on May 21.
Chase Utley had two RBIs for the Phillies.
NOTES: Marlins SS Hanley Ramirez was held out of the starting lineup with a sore back. ... Marlins RHP Josh Johnson has right shoulder inflammation and a strained mid-back, making it unclear when the All-Star will return to the mound. There was no structural damage found and he will rejoin the Marlins on Friday for a series in Washington, but the team has not decided when he might pitch again. ... Phillies closer Brad Lidge was unavailable because of a tender right elbow. Pitching coach Rich Dubee says he's "very confident" Lidge will be ready by Friday. ... Phillies rookie OF Domonic Brown is day-to-day with a strained right quadriceps.