Whole periods of his life - his youth and other interludes - remain unaccounted for, while details as to his origins and identity have proven to be remarkably elusive. There are plausible arguments to suggest that his family was from Croatia, possibly from the Karlovac-Delnice region. Highlighting a further riddle concerning his identity, Schneider and Tuksar both point out that Jarnovic's (or Giornovichi's) first names, Giovanni Mane, do not appear in any literature about him until 36 years after his death, namely in Schilling's Enzyclopadie of 1840. These names become the form most usually cited in encyclopaedic and biographical works from then onwards, often rendered today in their Croatian form as Ivan Mane Jarnović. It is "quite incredible," Tuksar has remarked, "that one of the leading musicians of 18th century Europe could have lived for 64 years without his name and surname ever being given anywhere, not even in his printed works, in their full and proper form." Besides Jarnović, several variations in the rendition of his surname also occur: Jarnowick, Jarnovick, Jarnovichi, Jarnowicz, Garnovik, Giarnovicki, Giernovichi. It seems likely that shifts in the spelling and pronunciation of his name occurred according to the country in which he was living or performing or publishing his works at any given time.
At least one source does exist which indicates his first and last names, namely a register entry recording the baptism of Jarnović's daughter Sophia, in London, in 1795. The same document throws light on yet another area of uncertainty - that of Jarnović's family. Whereas little was previously known of the fate of his daughters "Mimi" and "Sofie", something of their lives and subsequent history is now on record.