Matthew Tucker, 48, has been convicted at Birmingham Crown Court of two counts of rape, sexual assault and possessing indecent images.
Police said they began investigating after the teenage girl who was receiving mental health support, told a friend about the incident and police apprehended Tucker, of Great Barr, Birmingham, shortly after.
Over 2,200 screenshots were recovered from his phone and tablet with conversations between Tucker and his vulnerable victim plus a log detailing sexual activity with children aged eight- to 16-years-old.
Detective Inspector Joe Davenport from the Public Protection Unit said: “The girl was autistic and had mental health issues… she was clearly vulnerable and we believe Tucker exploited that for his own sexual gratification.
“He befriended the girl’s mum and moved into the family home while the girl was receiving specialist mental health support – that may have been a cynical move to get closer to the teenager.
“My officers conducted a thorough investigation and supported the girl throughout the case to the conclusion at court.
“I wish her all the very best for the future and hope she receives all the support she needs to get through what’s been a hugely traumatic time for her.
“I would encourage anyone who has been a victim of a sexual offence to come forward and report the matter to us so we can take action.”
The Jury at Birmingham Crown Court sentenced him to 18 years in prison and ordered that he sign the sex offenders’ register for life. The paedophile will need to serve 12 years behind bars before he can be considered for parole.