BOSTON — For Glen A. Doherty, being assigned to a high-stakes security detail in Libya was just one of over a decade’s worth of adventurous overseas missions he had participated in as a security contractor and a member of the Navy SEALs.

“He’s been overseas many, many times, defending this country,” said Mr. Doherty’s younger sister, Kate Quigley. “He was highly trained and really good.”

Mr. Doherty, 42, was with the American ambassador, J. Christopher Stevens, in a diplomatic compound in Benghazi on Tuesday when militants stormed the gates. He was killed, Ms. Quigley said, along with Mr. Stevens, Sean Smith and Tyrone S. Woods.

A native of Winchester, Mass., Mr. Doherty worked as a ski instructor in Utah in his 20s, and remarked that if he did not settle on a career path by the age of 30, he would join the SEALs.