Howard hit his team-leading third home run of the spring.
Hamels, the first of the Phillies' vaunted starting rotation to make this third spring start, walked the first two batters of the game and gave up two runs in the opening inning. The 27-year-old lefty followed with three perfect frames.
Justin Duchscherer, who appeared in just five games the last two seasons for Oakland, made his first start for Baltimore and pitched two scoreless innings.
The two-time All-Star, whose debut was delayed by soreness in his left hip, hadn't pitched since April 30 after hip surgery.