He went on to study at the University of Edinburgh and graduated with an undergraduate Master of Arts (MA) degree in 1943.

His academic career was interrupted in World War II when he was involved in cryptography at Bletchley Park.

After a period when he worked on geometry of numbers and diophantine approximation, he returned in the later 1950s to the arithmetic of elliptic curves, writing a series of papers connecting the Selmer group with Galois cohomology and laying some of the foundations of the modern theory of infinite descent.

Cassels often studied individual Diophantine equations by algebraic number theory and p-adic methods.

His advanced textbooks have influenced generations of mathematicians; some of Cassels's books have remained in print for decades.