James Macdonald Cassels was born in Penang, Malaya [later part of Malaysia] on 9 September 1924, the son of Alastair Macdonald Cassels and Ada White Cassels (nee Scott). His father was a bank manager of the Chartered Bank of India, Australia and China.

Cassels was educated at Rochester House School in Edinburgh, then St Lawrence College, Ramsgate and Trinity College Cambridge where he obtained an honours degree in physics. At Cambridge, Cassels met his first wife, Jane Lawrence (daughter of A W Lawrence the youngest brother of T E Lawrence and Professor of Classical Archaeology). They married in 1947 and had a son and a daughter. Jane Cassels died in 1977. Cassels married Analesia Theresa Bestman in 1986 (the marriage ended in divorce in 1989).

In 1944, Cassels was drafted into the government atomic bomb project (code named "Tube alloys research"). In 1946, Cassels began to study for his PhD and worked on the inelastic scattering of slow neutrons by polycrystalline solids and the investigation of the frequency spectrum of lattice vibrations in a crystal. Cassels left Cambridge in 1949 to take up a Research Fellowship at Harwell and worked on the 110-inch cyclotron. He remained at the Atomic Energy Research Establishment at Harwell as a Fellow and Principal Scientific Officer until 1953. He also became interested in the politics of science and advocated British participation in the establishment of a Centre Europeenne pour la Recherche Nucleaire (CERN). At Harwell he was involved with the experimental programme of the synchrocyclotron.

He moved to the University of Liverpool in 1953, initially as a lecturer, then Professor of Experimental Physics, a position he held from 1960-1982. Cassels was an Extraordinary Fellow of Churchill College, Cambridge from 1959-1960. After a brief period 1959-1960 as Visiting Professor at Cornell University, Cassels became Head of Department at Liverpool, a position he held from 1960-1974.

The energy crisis of 1973 raised the prominence of the issue of energy conservation, and Cassels began to promote the idea of the transportation of warm water from power stations and its use to heat buildings. Cassels was a member of the Marshall Committee (chaired by Walter Marshall) later known as the Combined Heat and Power Group of the Department of Energy from 1974-1979. His interest in energy issues continued after his retirement from Liverpool University in 1982 and he sent many letters to the press on this and other topics. Cassels suffered from manic depression and from 1977 was a diabetic. He set up a company, Igitur Ltd, to promote the use of 'left-handed sugars'.

He died on 19 October 1994.