The Blue Jays placed Janssen on the disabled list with a strained right forearm on June 16, and Saturday marked his first rehab outing.
In the 7-6 New Hampshire victory, Janssen threw 13 pitches in a 1-2-3 seventh inning.
"[Casey threw] a very good inning of work last night, a clean inning," Blue Jays manager John Farrell said. "He felt good after the game."
Janssen's injury interrupted what had begun as a solid 2011 season. In his first 22 outings, Janssen posted a 1.61 ERA and held opponents to a .200 batting average. In six June appearances, when his injury began hampering him, Janssen had an 8.44 ERA, while opponents hit .360 against him.
Janssen will make his second rehab appearance on Monday, but no further activity has been determined.
"There's no definite timetable yet," Farrell said. "Most importantly, he's answering the question of the health issue. That hasn't reoccurred. And, after his outing [on Monday], there's no doubt [Blue Jays GM] Alex [Anthopoulos] and I will speak and make a decision going forward on changes to the roster, changes to the staff."