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 has been credited for being the first to spread the term and concept of Big Data in the 1990s. He is now a consultant for venture capitalists and high-tech companies and a trustee of the Computer History Museum. In 2012, he received the USENIX Lifetime Achievement Award ("Flame Award") "for his contributions to the UNIX community since its early days".
He has written articles for the Skeptical Inquirer regarding climate change denial. In 2010 he published a 250-page critical report on the Wegman Report. Mashey's report concluded that the Wegman report contained plagiarized text. This story was featured in USA Today, and he was interviewed in Science magazine, which stated that he was "spending his retirement years compiling voluminous critiques of what he calls the 'real conspiracy' to produce 'climate antiscience'." His research has investigated the secretive funding of climate contrarian thinktanks. Mashey blogs at DeSmogBlog, which focuses on global warming.