Mashey joined MIPS Computer Systems in early 1985, managing operating systems development, and helping design the MIPS RISC architecture, as well as specific CPUs, systems and software. He continued similar work at SGI (1992 – 2000) most recently contributing to the design of SGI's NUMAflex modular computer architecture, ending as VP and chief scientist.
He later became a managing partner with Sensei Partners LLC, an early venture firm. Mashey was one of the founders of the SPEC benchmarking group, was an ACM National Lecturer for four years, has been guest editor for IEEE Micro, and one of the long-time organizers of the Hot Chips conferences. Additionally, he has chaired technical conferences on operating systems and CPU chips, and has given more than 500 public talks on software engineering, RISC design, performance benchmarking and supercomputing.
He holds a Ph.D. in computer science from Pennsylvania State University.