After the 1919 fall of the early Hungarian Soviet Republic, his mother and stepfather, Gertrúd Borstieber and György Lukács, left the country together, and moved to Vienna.
He worked in the laboratory of Werner Kolhörster in Berlin (1934–1936) focusing on astrophysics until he and his wife had to move again, fleeing Nazism.
In 1947 invited by Walter Heitler and Erwin Schrödinger he joined the Dublin Institute for Advanced Studies as a professor and group leader of the cosmic rays research laboratory.
Jánossy was charged with the task of managing the Cosmic Radiation Department at the Central Research Institute for Physics (Hungarian abbreviation: KFKI) founded in 1950.
[1] His name is linked to Geiger's coincidence detector development with special application to cosmic-ray secondary components created in the upper layers of the atmosphere (mesons such as kaons, muons, gamma rays).
But as the large accelerators started to take over the leading role, Jánossy turned away from the investigation of cosmic radiation and focussed on theoretical problems of quantum mechanics, the dual character of light, as well as the theory of relativity.
In the last one and a half decades of his theoretical activity, he was engaged in the hydrodynamic model of quantum mechanics and the interpretation problems of the theory of relativity.