Magnusson was first assistant from 1937 to 1940 and associate professor (
laborator) from 1940 to 1944 at the
Royal Swedish Academy of Sciences's Research Institute of Physics. He was administrative director from 1941 to 1944 and prefect from 1944 to 1945 at the
Military Physics Institute. The institute became the
National Defence Research Institute (FOA) in 1945 with Magnusson as head of the department (FOA 2; general physics 1945–58 and FOA 4; nuclear physics and nuclear chemistry 1958–68) from 1945 to 1968. In 1945, Magnusson was given the task by the board of FOA to investigate what was known about the nuclear device that was recently
dropped on Hiroshima and Nagasaki. It was the start of the
Swedish nuclear research. Magnusson became Director-General of FOA in 1968, a position he held until 1974. Magnusson was a member of the Swedish National Committee for Physics (
Svenska nationalkommittén för fysik) from 1946 to 1970 and a member of the Atom Committee (
Atomkommittén) from 1952. He was also a member of the Swedish Atomic Research Council (
Statens råd för atomforskning) from 1952 to 1965 and a delegate in atomic energy issues from 1956 to 1974 as well as in the Data Processing Delegation (
Databehandlingsdelegationen) from 1966. He was a board member of the
Defence Materiel Administration from 1968 to 1974, AB Atomenergi from 1969 to 1975 and the National Swedish Nuclear Power Inspection Board (
Statens kärnkraftinspektion) from 1974 to 1978. ==Personal life==