We begin with a discussion of the public launch of ENIAC in February 1946 as a carefully orchestrated scientific spectacle. We draw on oral history accounts and on archival sources, including seating plans, press releases, and records of the demonstration itself. We then consider ENIAC’s first year as an operational computer, during which its initial programming method was used on to run around a dozen applications at the Moore School. These are listed with brief discussion. We describe in some detail calculations performed by Douglas Hartree on supersonic airflow, the firing table calculations run as a public demonstration, and a mathematical experiment to detect prime numbers. These case studies integrate discussion of the various mathematical methods involved, the compromises and methods needed to adapt them for ENIAC, and the labor processes involved in running them on an often uncooperative machine in a chaotic environment.