He began working in business for Cushman Bros., which later became Lamont, Corliss, and Company, and turned it into a successful importing and marketing firm. It was an advertising agency that worked for food corporations. The company was in a bad financial status, but Lamont fixed it, and the company changed its name to Lamont, Corliss, and Company. He was partners with his brother-in-law, Corliss. His banking caught the attention of banker Henry P. Davison, who asked Thomas to join the new Bankers Trust. He started as secretary and treasurer and then moved up to being Vice President and then was promoted to director. He rose to the vice presidency of the First National Bank.