He was born in 1925 in Chippewa Falls, Wisconsin. He graduated from high school in 1943 before being drafted and seeing action in both Europe and the Pacific. On his return to America he took a B.Sc in Electrical Engineering at the University of Minnesota, graduating in 1950. He also gained a M.Sc in Applied Mathematics in 1951.
In 1950, Cray joined Engineering Research Associates in Saint Paul, Minnesota. Cray quickly came to be regarded as an expert on digital computer technology, especially following his design work on the ERA 1103, the first commercially successful scientific computer. He remained at ERA as it was bought by Remington Rand and then Sperry Corporation. There ERA became the "scientific computing" arm of their Univac division.
But when the scientific computing division was phased out in 1957, a number of employees, including Cray, left to form Control Data Corporation. By 1960 he had completed the design of the CDC 1604, a low cost computer that had impressive performance for its price range.
Even as the 1604 was starting to ship in 1960, he had already moved on to designing its "replacement", the CDC 6600. The 6600 was the first supercomputer, outperforming everything available by a wide margin. When other companies (namely IBM) attempted to create machines with similar performance, he simply upped the bar by releasing the 5-times faster CDC 7600.
This run finally ended in 1972 when Cray left Control Data, upset that they were no longer working on "large computers". There must be more to the story than just that, as he had been working on the latest machine in the series, the CDC 8600. A likely source of his frustration was likely to be the ongoing competing supercomputer project called Star.
However the split seems to be amicable, and when he started Cray Research a year later, CDC's CEO Willam Norris gave him $300,000 in start up money to get going. R&D and manufacturing were based in Chippewa Falls while the business headquarters were in Minneapolis. Ignoring the ideas of the CDC 8600 as too complex he went for a uniprocessor design - the Cray-1 (1976), the first was sold within a month to a lab in Los Alamos for $8.8 million.
In order to concentrate on design, Cray left the CEO position in 1980 to become an independent contractor. The successful Cray-2 (1985), with its innovative 3D circuit modules, led to the design of the Cray Y-MP and the Cray-3.
In 1989 the Cray-3 project was moved into a separate company, Cray Computer Corporation, headed by Seymour Cray and based in Colorado Springs, Colorado. However the 500MHz Cray-3 was a commercial failure. Heading for the 1GHz Cray-4, using gallium arsenide semiconductors, the company ran out of money and was forced to file for bankruptcy in 1995. Cray began working for SRC Computers but died of injuries suffered in an car accident on October 5, 1996 aged 71.
Beyond the design of computers Cray led a "streamlined life". He avoided publicity and there are a number of unusual tales about his life away from work. While he enjoyed skiing, wind surfing, tennis and other sports, another favorite pastime was digging tunnels.