Headley, considered the Padres' top Minor League prospect, joined the team in Cleveland on Sunday, though he wasn't activated until Tuesday.
Headley was in the starting lineup in left field and hitting sixth on Tuesday, as the Padres opened a three-game series against the Yankees at Yankee Stadium.
To create a spot on the 25-man roster for Headley, the Padres outrighted outfielder Justin Huber to Triple-A Portland.
Headley, the converted third baseman who is in his first season playing the outfield, was hitting .305 with 13 home runs and 40 RBIs in 65 games for Portland.
"He's very consistent, very level-headed," San Diego manager Bud Black said Tuesday. "He's got a good perspective of what's going on around him."
Headley, who had a .383 on-base percentage at Portland, is a switch-hitter who hit .330 with a .437 on-base percentage with Double-A San Antonio a year ago to earn Texas League MVP honors.
This season, Headley bounced back after a slow start (.242 batting average in April) by hitting .357 in May. He was able to cut down on his strikeouts, hit with more power and take more walks, which more or less matched his professional profile.
"I made some adjustments at the plate," Headley said. "In Triple-A, there is a lot more experience; guys know what they're doing a little bit more. I was seeing more offspeed [pitches] than I was before. It took me a little bit to make the adjustment to that, but once I figured out my approach, I got back to where I needed to be."
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.