One of America's premier railroad builders, Dillon began his career in the industry working as a water boy on the Mohawk and Hudson, one of America's earliest railroads. He was actively involved in the construction of numerous roads, his largest being the Union Pacific, with which he became actively involved in 1865 through a stock purchased in the Credit Mobilier. As one of the principal contractors for the Union Pacific, Dillon's vast experience in the construction of railroads proved invaluable. He took part in the laying of the last rail in 1869 receiving one of the ceremonial silver spikes used to complete the project. 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 Co.