Instructions were fed into the machine on paper tape, cards, or by switches. The numbers on which the instructions were to operate were stored in registers. Mark I was an electromechanical machine. Its basic operations were performed by mechanical parts which were controlled electrically by about 3,000 relays. Although obsolete by the time it went into operation at Harvard in 1944, Mark I operated for more than 15 years and produced valuable mathematical tables.

Aiken, who was familiar with the work of Babbage once remarked, "If Babbage had lived seventy-five years later, I would have been out of a job."

While Mark I was solving U. S. Navy math problems, plans were underway for the machine that was to make the breakthrough from automatic processing to electronic operation.