He was a Research Associate at Stanford Linear Accelerator Center 1969–74. Returning to Oxford he was a research officer (1974–78), teaching lecturer at Christ Church (1976–78), Senior Research Fellow at Merton (1977–79), and Fellow and Tutor at Balliol and University Lecturer in Physics (1979–90). He was appointed Reader in Experimental Physics in 1990 and Professor of Experimental Physics in 1991. He was also Head of Particle and Nuclear Physics 1991–96 and Chair of the Department of Physics 1997–99. He was appointed Principal of Brasenose from 2002 onwards. Cashmore also served as Director of Research and Deputy Director General at Organisation européenne pour la recherche nucléaire (CERN) from 1999 to 2004. During his term, several agreements took place with China's and Pakistan's to be of the most important. In 2002 he became co-chairman of the CERN-JINR joint scientific committee after Jim Allaby's retirement.