"I took a 6:25 train up here this morning and I'm on my way back" to Philadelphia, Rendell told reporters after his brief testimony. "There are very few people I would take a 6:25 train for."
Rendell was the first in a procession of character witnesses who testified Monday.
Later in the day, lawyers for the prosecution and defense made their closing arguments, and Judge Todd Hoover instructed the jury on the law. Jurors planned to begin deliberations Tuesday.
Stetler, 62, served as a state representative from York County for 16 years. The corruption charges against him involve the period between 2004 and 2006, when he was chairman of both the House Democratic Policy Committee and the House Democratic Campaign Committee.
He has pleaded not guilty to four counts of theft and one count each of conspiracy and conflict of interest.
Some of his former employees who testified for the prosecution said they routinely did campaign work on state time, including opposition research to expose the weaknesses of Democrats' political opponents. Stetler, testifying last week, denied the allegations and insisted that he instructed his employees "face to face" to keep public and political business separate.
Rendell, who was invited to testify by Stetler's daughter, said Stetler never should have been charged.
"I think that this prosecution was a mistake," he told reporters, citing "fairly clear" evidence that Stetler did not break the law. But he acknowledged: "I'm not here to be a fact witness."
Stetler served as Rendell's revenue secretary for about 11 months until he resigned hours before he was charged in 2009.
He was among the last of 25 former legislators and ex-aides who were charged in the state Attorney General's Office's investigation of government corruption. Twelve Democrats and nine Republicans have been convicted or pleaded guilty. Two defendants were acquitted, and charges against a third were dropped.