Jonnjoined the green army commanded by Lord Ormund Hightower and fought in favour of King Aegon II Targaryen. Jon fought in the Battle on the Honeywine, where he used his sword to mortally wound Lord Owen Costayne, who had sided against his liege lord with the blacks.
Jon presumably fought in the First Battle of Tumbleton and afterwards he was a vocal member of the caltrops conspiracy, which sought the deaths of Hugh Hammer and Ulf the White due to their outrageous demands of lordships and kingships, and possibly also because of the death of Ser Roger Corne.
Jon killed Hugh as he ran for his dragon, Vermithor, cutting him from groin to throat with Orphan-Maker. He would then fight some of Hugh's men, killing three before then being overpowered and killed himself. Some say his death came because he slipped on Hugh's entrails.