Smith worked for his family's business, Cleary Brothers Inc., which operated tugs and barges on New York's upstate canals and in New York Harbor. He then headed the Park Agency Inc., in Manhattan, where he managed $300 million worth of Kennedy family investments.
After his father-in-law Joseph P. Kennedy Sr. suffered a stroke on December 19, 1961, Smith, still working in Manhattan, managed the family's fortune. He was responsible for overseeing the trusts that benefited him and his family as well as the other children and grandchildren of Joseph and Rose Kennedy.
Smith and Jean Ann Kennedy were married May 19, 1956, in St. Patrick's Cathedral in Midtown Manhattan, at which time he became the brother-in-law of future President John F. Kennedy. Stephen and Jean had two biological sons and later adopted two daughters: