His parents divorced when Mr. Robertson was a boy, and he was raised by his father’s second wife, Zelda (Ziegel) Robertson, also a homemaker. In 1945, when he was 17, Leslie lied about his age and joined the Navy. He was not deployed, and he was honorably discharged that September. He studied engineering at the University of California, Berkeley, receiving a bachelor of science degree in 1952.
Over the next six years he worked as a mathematician, an electrical engineer and a structural engineer; for a time, while living in New York, he investigated the collapse of an offshore drilling platform. When that job ended, he decided to head west to California with his family in a Volkswagen convertible. The money ran out in Seattle, and in 1958 he took the first job he could get, at Worthington and Skilling, a structural engineering firm. Its clients included the Seattle-born architect Minoru Yamasaki, who had several projects in that city.
In 1962, Mr. Yamasaki won a competition to design the World Trade Center, and he helped Mr. Robertson’s firm obtain the engineering contract. “What that man did to me was incredible,” Mr. Robertson said. “I was a kid, and he said, ‘Go for it.’”
He added: “We had never done a real high-rise project before.”
Mr. Yamasaki felt that tall buildings were uncomfortable to be in unless they provided a sense of enclosure. It was that notion that led to the tube design, with exterior columns about two feet apart for most of the buildings’ height.
Mr. Robertson moved to New York to work on the Trade Center; Mr. Skilling stayed in Seattle. (He died in 1998.) In 1982, the firm — by then known as Skilling, Helle, Christiansen, Robertson — broke up, and its New York office became Leslie E. Robertson Associates, later LERA. Mr. Robertson gave up his partnership in 1994 but worked on the firm’s projects until 2012.
His first two marriages, to Elizabeth Zublin and Sharon Hibino, ended in divorce. He married SawTeen See, an engineer and later managing partner of LERA, in 1982. She survives him. In addition to her and his daughter Karla, from his first marriage, he is survived by a son, Chris, from his first marriage; a daughter, Sharon Robertson, from his second marriage; and two grandsons. Another daughter from his first marriage, Jeanne Robertson, died of breast cancer in 2015.