Thomas Durant graduated from Albany Medical College, but he never practiced medicine, choosing instead to devote his career to business.
After a period of working in his uncle's grain and flour export business, he discovered the need for improved inland transportation, a discovery that led him to the railroad industry. Durant got his start in the railroad industry working as a broker for the Chicago and Rock Island Railroad. It was during this time that Durant became professionally acquainted with Henry Farnam.
The two men created a new contracting company under the name of Farnam and Durant. In 1853 they were given the commission of raising capital and managing construction for the newly chartered Mississippi and Missouri Railroad (M&M). In 1862 Durant negotiated a contract with the U.S. government to build the Union Pacific. Durant joined the company as vice president and general manager mainly in order to protect and extend his own financial interests.
One of Durant's biggest coups was the creation of Credit Mobilier of America. Durant and George Francis Train joined together in March 1864 in a business venture to buy out the Pennsylvania Fiscal Agency, changing its name to Credit Mobilier. The company was one of the first to take advantage of the new limited liability financial structures. Previously investors were responsible for the finances of a company if it had problems. Under limited liability, their only responsibility was for money paid in. Credit Mobilier was created to build the railroad track. Durant manipulated its structure so that he wound up in control of it. His company Union Pacific was effectively paying him via Credit Mobilier to build the railroad. Durant covered his tracks by having various politicians, including future President James Garfield, as limited stockholders. In 1876 Durant was ousted from his position managing Credit Mobilier.