Prior to joining Microsoft, he worked for Heathkit, on HDOS and Benton Harbor Basic.
While at Microsoft, Letwin worked on a number of projects. His first project at Microsoft was writing a BASIC compiler. He is most noted for being the lead architect of the OS/2 operating system on the Microsoft side, with Ed Iacobucci being the lead architect from IBM's side. Letwin contributed much of the design and code for several core components, including the HPFS file system.
Letwin left Microsoft in 1993 to "kick back" with his wife. While at Microsoft he had become a millionaire, with a 2000 TIME article estimating his worth at around $20 million. Since leaving Microsoft, Letwin has donated substantial amounts of money to environmental causes via the Wilburforce Foundation, a charitable foundation created by him and his wife, Rose.
- "25 Years Ago At Microsoft". TIME. May 1, 2000. Retrieved 2006-09-16.
- Zachary, G. Pascal (1994). Showstopper! The Breakneck Race to Create Windows NT and the Next Generation at Microsoft. Warner Books. ISBN 0-02-935671-7.
- Associated Press (April 12, 2000). "A look at Microsoft's first 11 employees". Boston Herald. Retrieved 2007-02-21.
- Letwin, Gordon (1988). Inside OS/2. Microsoft Press. ISBN 1-55615-117-9.