A 36-year-old army reservist from Essex has been found guilty of eight rapes, five counts of child abduction and one of grooming a girl.
Adrian Rose, of Rochester Drive in Westcliff-on-Sea, denied all the charges at Norwich Crown Court.
Rose was found in February 2012 in his car at the Broadland Business Park in Norwich with a 12-year-old girl who said she had had sex with him.
He was found guilty on Thursday and will be sentenced on 1 February.
Police were checking Rose's car because of concerns about the number plates and appearance and found the girl who was there without her parents' knowledge.
When Rose lied about her age they decided to take her home and during this journey the girl said she had met him through Blackberry Messenger and they had had sex.
Rose had been released but was later arrested in Southend and brought to trial accused of being a serial sex attacker.
Det Con Steve Graves, said: "Rose is a predator who has denied his actions throughout, which meant the girl had to relive the traumatic events which have happened to her in order to bring him to justice."