Christian Norris, aged 52, of Meadway, Buckingham, was sentenced to 17 years and six months in prison at Aylesbury Crown Court on Wednesday (7/9).
He will also be on the sex offenders register for life.
Norris pleaded guilty to four counts of assault of a child by penetration, three counts of making indecent photographs of a child, one count of causing or inciting a child to engage in sexual activity and one count of sexual assault of a child at the same court on 9 August.
The conviction is in relation to incidents which occurred between June 2007 and May 2021.
Norris was charged on 18 June this year.
Investigating officer Detective Constable Victoria McEwen, based at Milton Keynes police station, said: “A very dangerous man has been sentenced, and will now serve a substantial prison sentence. His horrendous crimes were committed against the most vulnerable in our society.
“I would like to thank the victim and the victim’s family for their support throughout this investigation and hope the outcome will provide some closure for them.
“I hope Norris uses his time in prison to reflect on his wholly unacceptable behaviour.
“Offences such as this will always be treated very seriously by Thames Valley Police and we will always look to investigate and prosecute offenders accordingly.”