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 he joined Engineering Research Associates in St. Paul, Minnesota, helping design the earliest digital computers. 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. But when the scientific computing division was phased out in 1957 he left to become a early employee of Control Data Corporation. By 1960 he had completed the design of the Control Data 1604, the first fully transistorized computer and had begun the design of the CDC 6600. By 1970, Cray was designing systems that were clear indicators of the high performance computers yet to come. In the design of the CDC 8600 he had moved beyond improving pure clock speed to seeking much greater parallelism and he designed the 8600 with four processors all sharing one memory and was moved towards what would later be called RISC.
Cray left Control Data Corporation in 1972 to establish his own company - Cray Research. 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-3. However the 500MHz CRAY-3 was a commercial failure and in 1989 Cray Research was bought by SGI. The CRAY-3 project was moved into a separate company, Cray Computer Corporation, headed by Seymour Cray and based in Colorado Springs, Colorado. 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.