Mejia was activated from the restricted list before Tuesday night's game between the Mets and San Francisco Giants at AT&T Park. The right-hander, who pitched in a rehab game on Sunday, was in uniform and available to play.
Mejia was suspended in April after testing positive for the steroid Stanozolol. The 25-year-old pitcher has said he never knowingly took the steroid.
To clear room on the 40-man roster the Mets transferred Buddy Carlyle to the 60-day disabled list.