Together with John W. Mauchly he constructed the ENIAC, sometimes considered the first digital computer (but see John Vincent Atanasoff for conflicting claims), from 1941-1945. Mauchly concentrated on the overall design while Eckert constructed the electronic circuits.
Both Eckert and Mauchly left the Moore School at the University of Pennsylvania in October 1946. They started up the Electronic Control Company which built the Binary Automatic Computer (BINAC). One of the major advances of this machine, which was used from August 1950, was that data was stored on magnetic tape rather than on punched cardss.
Electronic Control Company soon became the Eckert-Mauchly Computer Corporation and it received an order from the National Bureau of Standards to build the Universal Automatic Computer (UNIVAC). In 1950, Eckert-Mauchly Computer Corporation ran into financial troubles and was acquired by Remington Rand Corporation. The UNIVAC I was finished in December 1950.
Eckert remained with Remington Rand and became an executive within the company. He continued with Remington Rand as it merged with the Burroughs Corporation to become Unisys in 1986. In 1989, Eckert retired from Unisys but continued to act as a consultant for the company.