The title of the thesis was Dekompozicija nekih klasa nedegeneriranih Rédeiovih grupa na Schreierova proširenja (Decomposition of some classes of nondegenerate Rédei Groups on Schreier extensions), in which he solved a problem posed by László Rédei.
[2][3] He then taught physics at a high school in Široki Brijeg in Bosnia and Herzegovina.
[2] In 1962 Janko decided to leave Yugoslavia for Australia, where he first taught at Monash University in Melbourne.
[2] Janko discovered his first sporadic simple group (called J1) in 1964, when he was at the Australian National University.
Finally, Janko found the group J4 in 1975; its existence was confirmed in 1980 by Simon P. Norton and others using computer calculations.