In 1940, Durst joined the real estate firm, the Durst Organization, which had been founded by his father, Joseph. After Joseph's death in 1974, Seymour took control of the Durst Organization. The company invested in Manhattan real estate, based upon Durst's belief that one should never buy anything one cannot walk to.
Durst was vocal about his beliefs that the government should not interfere in real estate transactions. He was also concerned with the ballooning national debt. In 1989, Durst created and installed the National Debt Clock on a Durst Organization property in order to draw attention to the then $2.7 trillion debt.