With his brother, Paul, he presided over a multibillion-dollar real estate and banking empire with three million square feet of office space, 8,000 apartments and one of New York's oldest financial institutions, the Emigrant Savings Bank.
His father, Morris, had founded Circle Floor Company, a floor and ceiling contractor that installed the floors at Rockefeller Center the World Trade Center and other buildings. In 1941, Seymour Milstein graduated from New York University, and in 1945, he married Vivian Leiner, who survives him, as do six grandchildren and his children, Constance and Philip.
In 1970, the family took control of United Brands, the food company, and Starrett Housing Corporation. They later sold the companies for a profit. In 1986, they took over the failing Emigrant Savings Bank and pumped $90 million into it.
In the 1960's, Paul Milstein built the family's first apartment house, the Dorchester Tower near Lincoln Center. In the 1980's the Milsteins built Normandie Court and Windsor Court on the Upper East Side and the Liberty buildings in Battery Park City.
During their partnership, Seymour Milstein handled the financial details and was in charge of dealing with banks.