After earning his degree, Casden took a position with Kenneth Leventhal & Company at the urging of one of his professors. During this period, Kenneth Leventhal became the major accounting firm for large U.S. residential real estate developers, which exposed Casden to a variety of major real estate entities that eventually became his clients. Casden excelled during his time with Kenneth Leventhal, founding the company's San Francisco office in 1969. In 1975, Casden accepted a job with Los Angeles apartment developer Robert Mayer and with financial support from the Belzberg family of Canada, he was able to take control of the firm. In 1980, he bought out the Belzberg interest after a dispute regarding future strategic directions the renamed Casden Properties, LLC, based in Beverly Hills. Using obscure incentives and deductions in the United States Tax Code, Casden was to profitably develop low-income housing projects and within a few years, Casdan properties had become one of the largest multi-family developers in the United States, winning 30 major design awards and earning recognition from the New York Times as an example of the use of urban planning to prevent urban sprawl. In 2002, Casden sold the majority of his real estate portfolio to the Denver real estate investment trust (REIT), Apartment Investment & Management Company (AIMCO) for $1.1 billion in cash, AIMCO stock, and assumed debt.