Mr. Simons began his career as a popular professor at MIT and Harvard University. During the Cold War, he broke Russian code working for an organization aiding the National Security Agency. At 37, while running Stony Brook University’s math department, he won geometry’s highest honor, cementing his reputation in mathematics. Friends at the time said Mr. Simons’s rugged, craggy features and the glint of mischief in his eyes reminded them of Humphrey Bogart.

When Mr. Simons left Stony Brook in 1978 to launch his trading firm, he was eager for a new challenge and bursting with self-confidence.