Khan, 48, was expelled from the Conservative Party and resigned, triggering a by-election in Wakefield, West Yorkshire, after he was found guilty last month following a trial.
The jury heard how he forced the then teenager to drink gin and tonic and asked him to watch pornography, before the attack at a house in Staffordshire following a party in January 2008.
The victim, now 29, said he was left feeling “scared, vulnerable, numb, shocked and surprised”, after Khan, then 34, touched his feet and legs, coming within “a hair’s breadth” of his privates, as he went to sleep in a top-bunk bed.
Khan, who denied sexual assault, was jailed for 18 months at Southwark Crown Court on Monday by Mr Justice Baker.
The judge said he had shown no remorse, adding: “The only regret you feel is towards yourself for having found yourself in the predicament you face as a result of your actions some 14 years ago.”
Khan showed no emotion as he was taken down to the cells, carrying, among his possessions, a bag from an upmarket men’s grooming shop.
Imran Ahmad Khan arriving at Southwark Crown Court in London
Mr Justice Baker said he had displayed a “significant degree of brutality” in the lead up to the assault, as he dragged his victim upstairs and threw him on to a bed.
“I am satisfied that the complainant was particularly vulnerable, and not only was he 15 years of age at the date of the offence, but I accept his mother’s description that he was not very worldly and very young for his age,” he said.
“Although it may well be, over the years, you had let yourself believe you had got away with having committed this offence, I am sure you were aware from the outset there was a risk there would be a day of reckoning.”
Khan has said he will appeal against his conviction, and Gudrun Young QC, defending, said on Monday: “Mr Khan denies this offence and maintains today he is not guilty of it.”
Reading an impact statement in court, his victim said he has had “suicidal thoughts” and suffered difficulties in his relationship and work.
“Because of this assault, throughout my teenage years I found being touched in any way difficult,” he said.
“My mental health has deteriorated rapidly since deciding to come forward and having to constantly relive an event I tried to bury for such a long time,” he continued.
Imran Ahmad Khan
“I have struggled with the guilt of dragging my family back into a horrible ordeal they would rather forget, and watch them struggle with their own guilt for allowing that man into the house.”
The court heard the victim was “inconsolable” when he ran to his parents following the attack.
A police report was made at the time, but no further action was taken because the boy did not want to make a formal complaint.
However, he told jurors “it all came flooding back” when he learned Khan was standing in the December 2019 general election.
He said he was not “taken very seriously” when he made the allegation to the Tory press office days before Khan won the Wakefield seat, and went to police after Khan helped Prime Minister Boris Johnson win a large Commons majority.
Khan, who is gay and a Muslim, claimed he only touched the Catholic teenager’s elbow when he “became extremely upset” after a conversation about his confused sexuality.
He said he was trying to be “kind” and “helpful”, but the boy became upset, and “bolted” when the topic of pornography was raised.
However, Khan was found guilty of a single count of sexual assault by a jury after about five hours of deliberations.
His barrister said Khan, who has a degenerative bone disease and is registered disabled, has been “completely teetotal and celibate for a number of years”, and has “become somewhat of a recluse,” living with and caring for his elderly mother.
In mitigation, Ms Young said that since his election to Parliament his life has been in “freefall”, adding: “At every stage of this process, the investigation, charge, trial, everything he has worked for and achieved in his life has been gradually stripped away from him, ending in this conviction.”
She said he had gone from “high public office”, with a successful career behind him, to being “utterly and completely disgraced, with his life and career in ruins, shamed and humiliated at every turn”.
“To say his reputation is in tatters does not do the matter justice. It has been completely destroyed,” she said.
“Mr Khan’s fall from grace has been spectacular.”
She added: “He will always be known as a disgraced former MP and he will take that to his grave.”