Thatcher, two months and two days removed from surgery on his left shoulder, threw off a mound for the first time since February in Spring Training.
The throwing session was an important step in his rehabilitation, mentally and physically.
"This was sort of my last mental hurdle ... getting on the mound again," Thatcher said. "It was exciting and just fun to be back out there, throwing and visualizing hitters."
Thatcher threw 25 pitches, all fastballs, to bullpen catcher Justin Hatcher. Manager Bud Black and pitching coach Darren Balsley watched from the side.
It was May 2 when Thatcher had surgery to shave down his frayed labrum and rotator cuff and to remove the bursa sac in his left shoulder.
Thatcher was sidelined most of Spring Training because of soreness in his left shoulder and then started the season on the 60-day disabled list, retroactive to March 22.
Before Monday, the last time Thatcher stood on a mound was in late February in Spring Training while throwing batting practice to Padres hitters. That's when he first started to feel pain in his shoulder.
Thatcher will throw another bullpen session Wednesday as he progresses during his rehabilitation that will eventually lead him to side sessions and then some time pitching in the Minor Leagues on a rehabilitation stint.
"That's a big step. ... In a sense, this is his Spring Training," Black said. "There are still a number of hurdles he has to clear but this [Monday] was a big one."
There's no timetable to when Thatcher -- who last season was one of the top left-handed specialists in the National League, posting a 1.29 ERA in 65 games -- will return, nor are the Padres anxious to push for a speedy return.