Crédit Mobilier of America was a company set up by the Union Pacific to defraud United States taxpayers in the construction of the First transcontinental railroad.

The result was the Crédit Mobilier of America scandal, which exposed an over-invoicing and a stock and bond share-pricing scheme, whereby Union Pacific officers and directors, including Dillon, profited by manipulating the share price of Crédit Mobilier of America's stock shares and bonds, padding invoices to the U.S. Government, and bribing congressmen with shares in Crédit Mobilier of America, cash and other perks.

As one of the principal contractors for the Union Pacific, Dillon's vast experience in the construction of railroads proved invaluable.

Following 1870, Dillon was primarily known as a financier, becoming involved with Jay Gould in numerous ventures as well as serving on the board of directors of the Western Union Telegraph Company.

[2][3] Dillon died at his home at 23 West 57th Street in New York City, after a twelve-week illness, at the age of 80.