His exit is at least the second of a relatively high-ranking portfolio manager since July 25, when SAC was indicted by the government on charges of securities and wire fraud.
In August, Anil Stevens—the trader who ran a financials-stock group within the firm, called Parameter Capital—departed along with his team as the entire unit was shuttered. And other midlevel analysts and marketers have also either resigned or been let go as part of SAC's selective downsizing.
(Read more: SAC sees traders bolt as firm's troubles mount)
SAC said it will fight the criminal charges against it, as well as a separate charge against founder Steven Cohen for failing to supervise staff members convicted or accused of misconduct. The firm is in talks with prosecutors over a possible settlement of the criminal charges, according to someone familiar with the matter.
In a written statement issued late Monday, an SAC spokesman thanked Tiller for his 12 years at SAC. Members of the firm "wish him well in his retirement and expect that he will make Sustainable America as successful" as his career at the Stamford, Connecticut hedge fund, the statement read.
"I will be devoting most of my time to Sustainable America," Tiller said in a short e-mail statement.
—By CNBC's Kate Kelly