Cowlishaw was a pre-University student in 1971 and joined IBM in 1974 as an electronic engineer but is best known as a programmer and writer.

He is known for designing and implementing the Rexx programming language (1984),[2][3] his work on colour perception and image processing that led to the formation of JPEG (1985),[4] the STET folding editor (1977), the LEXX live parsing editor with colour highlighting for the Oxford English Dictionary (1985),[5] electronic publishing, SGML applications, the IBM Jargon File IBMJARG (1990),[6] a programmable OS/2 world globe PMGlobe (1993),[7] MemoWiki based on his GoServe Gopher/http server,[8] and the Java-related NetRexx programming language (1997).

He has contributed to various computing standards, including ISO (SGML, COBOL, C, C++), BSI (SGML, C), ANSI (REXX), IETF (HTTP 1.0/RFC 1945), W3C (XML Schema), ECMA (JavaScript/ECMAScript, C#, CLI), and IEEE (754 decimal floating-point).

Cowlishaw has worked on aspects of decimal arithmetic; his proposal for an improved Java BigDecimal class (JSR 13) is now included in Java 5.0, and in 2002, he invented a refinement of Chen–Ho encoding known as densely packed decimal encoding.

[9] Cowlishaw wrote an emulator for the Acorn System 1, and collected related documentation.