Jonke was born in Karlovac, where he completed primary school and Gymnasium Karlovac. He graduated at the Faculty of Philosophy at the University of Zagreb the history of Yugoslav literatures, Croatian and Old Church Slavonic language and folk history with Russian and Latin. He spent two years at the Charles University in Prague. Demonstrating the affiliation to literary and historical topics, he starts to translate from Czech. From 1933 he worked as a professor at the gymnasium in Sušak, and in 1940 he relocated to Zagreb, where professor Stjepan Ivšić chose him as an assistant in 1942. He was married to Nada Marković in 1940 with whom he had a daughter Dubravka and son Mladen. Simultaneously engaging himself in the topics of Croatian and Czech studies, he received his Ph.D. with a thesis Dikcionar Karlovčanina Adama Patačića. From autumn 1945 he taught Czech language and literature, and from autumn 1949 modern Croatian language at the newly established department which he was a head from 1950, when he acquired the status of docent, up until the retirement in 1973. He became a regular professor in 1960. In the 1950s, Jonke engaged in systematic study of a completely neglected subject of problems of Croatian language from the Illyrian times to the end of 19th century. He carefully examined the work of Bogoslav Šulek and Adolfo Veber Tkalčević. Jonke was a participant of the 1954 Novi Sad agreement, styliser and the editor of "common" orthography, and one of the editors of the Dictionary of Serbo-Croatian literary language. As soon as the applications of Novi Sad conclusions became detrimental at the expense of Croatian language, he published a series of polemics with Serb linguists and writers in which he defended the right of the Croatian nation to its own language and the right of that language to achieve equal social status. He edited two columns of language advice, and edited the journal Jezik for 17 years. From 1970 to 1971, he served as the President of Matica hrvatska. In 1963 he was elected as a member of JAZU. Due to his alleged "Croat nationalism", having been denounced after signing the 1967 Declaration on the Status and Name of the Croatian Standard Language, and after the coup in Karađorđevo, he was forcefully retired in 1973. Since then he worked on the completion of the JAZU dictionary. Jonke's main contribution is in revitalising the interest to the contributions of Zagreb philological school and its essential role in the standardisation of Croatian language. He died in Zagreb.
