Janjanin was born in Carlovac, Croatia (then part of Yugoslavia) on January 18, 1957. He played for Dinamo Zagreb (1975-1980) and Red Star Belgrade (1980-1984) before coming to Greece to join OFI. At OFI, Janjanin was an instant success. Over the next three seasons he appeared in 50 Alpha Ethniki matches scoring 13 goals. He was acquired by AEK in December 1986 - despite OFI's best efforts - but did not have the success he had enjoyed at OFI and left after the 1988 season.
Janjanin was capped twice by Yugoslavia.