Rotblat was born in Warsaw in 1908 and obtained an MA from the Free University of Poland and a doctorate from the University of Warsaw. He moved to Liverpool in 1939, where he worked with James Chadwick. At Liverpool he started to work on the atomic bomb and later moved to Los Alamos. However, he resigned and returned to Liverpool after Germany had surrendered and he could see no justification for continuing to work on an atomic bomb.

He stayed in nuclear physics until 1950, when he moved to Saint Bartholomew's Hospital in London and established a new career for himself as a medical physicist. He was professor of physics at Bart's from 1950 until his retirement in 1976. He also served as editor of the journal Physics in Medicine and Biology and was president of both the Hospital Physicists' Association and the British Institute of Radiology.

However, Rotblat was best known for his opposition to nuclear weapons and his role in Pugwash. Named after a small town in Nova Scotia, Canada, where the first meeting was held, Pugwash brought together scientists from both sides of the iron curtain and its meetings are credited with playing a role in ending the cold war.

In his later years Rotblat's attention moved from nuclear disarmament to the elimination of the causes of all wars. As he wrote in the millennium issue of Physics World in December 1999: "This is truly a task fit for the next century."