In 1968, Fossett received an MBA from the Olin School of Business at Washington University in St. Louis, Missouri, where he was later a longtime member of the Board of Trustees. Fossett's first job out of business school was with IBM; he then served as a consultant for Deloitte and Touche, and later accepted a job with Marshall Field's. Fossett later said, "For the first five years of my business career, I was distracted by being in computer systems, and then I became interested in financial markets. That's where I thrived."
Fossett then became a successful commodities salesman in Chicago, first for Merrill Lynch in 1973, where he proved a highly successful producer of commission revenue for himself and that firm. He began working in 1976 for Drexel Burnham, which assigned him one of its memberships on the Chicago Board of Trade and permitted him to market the services of the firm from a phone on the floor of that exchange. In 1980, Fossett began the process that eventually produced his enduring prosperity: renting exchange memberships to would-be floor traders, first on the Chicago Board Options Exchange.
After 15 years of working for other companies, Fossett founded his own firms, Marathon Securities and Lakota Trading, from which he made millions renting exchange memberships. He founded Lakota Trading for that purpose in 1980. In the early 1980s, he founded Marathon Securities and extended that successful formula to memberships on the New York stock exchanges. He earned millions renting floor trading privileges (exchange memberships) to hopeful new floor traders, who would also pay clearing fees to Fossett's clearing firms in proportion to the trading activity of those renting the memberships. In 1997, the trading volume of its rented memberships was larger than any other clearing firm on the Chicago exchange. Lakota Trading replicated that same business plan on many exchanges in the United States and also in London. Fossett would later use those revenues to finance his adventures. Fossett said, "As a floor trader, I was very aggressive and worked hard. Those same traits help me in adventure sports."
Fossett said he did not participate in any of the "interesting things" he had done in college during his time in exchange-related activities: "There was a period of time where I wasn't doing anything except working for a living. I became very frustrated with that and finally made up my mind to start getting back into things." He began to take six weeks a year off to spend time on sports and eventually moved to Beaver Creek, Colorado in 1990, where for a time he ran his business from a distance. Fossett later sold most of his business interests, although he maintained an office in Chicago until 2006.