Farnsworth was brought in in the top of the eighth with the Astros leading the Chicago White Sox 6-3. Farnsworth gave up a run-scoring sacrifice fly to Alejandro De Aza and an RBI single to Gordon Beckham to cut the lead to 6-5.
The New York Mets dropped Farnsworth from their major league roster earlier this week -- days ahead of a deadline in which his $1 million salary would have become guaranteed. They instead were only responsible for a quarter of that amount.
Farnsworth, 38, went 0-3 with a 3.18 ERA and three saves in 19 appearances with the Mets this season.
He had agreed to an "advance consent" agreement that allowed the Mets to dump him within 45 days of being added to the roster and avoid paying the full-year salary.
Upset with how the Mets handled dropping him on Wednesday night, Farnsworth vowed revenge.
"Hopefully find a team to play against this team," Farnsworth said. "I'm very bitter right now."
The Astros visit the Mets at Citi Field in the final series of the season.
Information from ESPNNewYork.com's Adam Rubin was used in this report.