From A Wiki of Ice and Fire
|Book||Fire & Blood (mentioned)|
Ser Jon served as a Kingsguard during Maegor's reign. After Maegor's mysterious death, the new young king Jaehaerys I Targaryen offered Maegor's surviving Kingsguard a choice between execution or taking the black. Jon chose the latter.
- Fire & Blood, Prince into King - The Ascension of Jaehaerys I.