A jury found Mathew Martoma guilty of insider trading Thursday, handing prosecutors their eighth win against someone who worked at SAC Capital Advisors LP and possibly bolstering a related case against firm founder Steven A. Cohen.

Prosecutors have long pursued Mr. Cohen, who built one of the country's most successful hedge funds over the past two decades and managed more than $15 billion at the firm's peak. He has denied involvement in any wrongdoing and hasn't been charged criminally, but he faces a civil allegation by...