The 32-year-old left-hander threw 45 pitches for around 15 minutes in 89-degree weather.
"I felt good overall. I think all the work that we have been doing is paying off," Santana said. "I think the way I'm feeling right now is, I'm getting closer."
Whether Santana can begin a Minor League assignment on Wednesday will come down to how he feels on Monday.
"As of right now, I don't know," he said. "I just have to make sure that I feel good tomorrow, and that decision will be made in the next couple of days."
When he threw on Sunday, Santana -- who has been rehabbing at the Mets' Spring Training facility in Port St. Lucie, Fla. -- stepped on a big league mound for the first time.
The southpaw has missed the entire season as he recovers from surgery to repair an anterior capsule in his left shoulder.
"It's always good to be back and around the guys and the mound, the ballpark and everything," he said. "At the same time, I'm focused on making sure I'm getting healthy. It's definitely a different ballpark than what I've been doing over in the Minor Leagues, but it doesn't change what you do."
Asked how his bullpen session compared with those in past years, Santana said he thought it was better.
"[But] I'm not trying to compare to past years," he said. "Right now we're doing everything. We want quality. We want to make sure I feel good and command. I'm able to throw all my pitches without and problems."
Randy Niemann, Santana's rehab pitching coach, has been with the two-time Cy Young Award winner during his recovery process and has seen him get better each time out.
Niemann, who was the Mets' bullpen coach last season and pitched for the organization in 1985 and 1986, has noticed the progress Santana has made.
Three months ago, Santana couldn't toss at 60 feet for more than 25 throws.
"What a difference. He's made great progress," Niemann said. "I'm very proud of how hard he's worked. Every time I see him on the mound, I see just a little bit better than the last time, and that's what we're looking for."
Though the southpaw felt soreness in his left shoulder during a rehab assignment on June 3, he never stopped throwing, opting for long-toss and playing catch.
And as for Wednesday's Minor League outing, "It's going to depend on how he feels tomorrow," Niemann said. "When he does get to a game, whenever that is, it's going to depend on how he recovers from that. We'll take it one step at a time. We can't get ahead of anything, that's for sure."
Christina De Nicola is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.