A panel at the Central London Employment Tribunal discovered the 58-year-old “victimised” the woman as well as subjecting her to “unwanted conduct of a sexual nature”.
A judgment published on Wednesday said Mr Hill was accused of sexually harassing and bullying the claimant, referred to as Ms A, over a 16-month period when he was in office.
Mr Hill, who was an MP from 2017 until he resigned last year, has denied the allegations.
In her witness statement included in the judgment, Ms A said Mr Hill sexually assaulted her on a number of occasions.
The claimant said he got into bed with her in December 2017 and rubbed his genitals “against my bottom and had his arms wrapped around me and was feeling my breasts”.
She said: “I managed to get out of the bed and went to the living room. I was crying and shaking because of the experience.”
She said Mr Hill told her she was “overreacting”, must be “frigid or something” and there must be “something wrong” with her.
The judgment said Mr Hill was also accused of coming into the woman's bedroom on several occasions from December 2017 to February 2018.
Mr Hill is also said to have sexually assaulted Ms A on a number of occasions in his Westminster office.
Ms A said in her witness statement: “He often put his arms around the front of my body and brushed his hands against my breasts.
“If I were standing up, he would always approach me from behind and he would hold me by my waist. I would always resist and would try to get him off me but did not always succeed.”
She added: “Mr Hill would touch my bottom. When he first did this, I asked him not to do it again but, as he did then (and afterwards), he always played it down as if it was my problem and I was making more of it than I should.”
The judgment also said Mr Hill terminated the claimant's employment.
The panel also found that he failed to give the claimant a promised pay rise, made her work through the summer recess despite a previous promise she would not have to, and refused to let her travel to work by car unless she submitted an occupational health assessment. The politician was also found to have not replied to emails and texts which the claimant sent about work.
The judgment said the panel also heard that Ms A told Tory MP Andrew Bridgen of her allegations of sexual impropriety against Mr Hill.
Mr Bridgen then accused Kate Hollern, a Labour MP, of warning him not to get involved in the harassment case.
Ms Hollern quit as shadow minister in May 2021 after an employment tribunal heard that she tried to intimidate Mr Bridgen as a witness.
In a previous statement, she said: “I am absolutely clear that any complaint of sexual harassment should be treated extremely seriously and had this been raised with me I would have taken the necessary action.
“It was never my intention to undermine the support the complainant was receiving, which I was unaware of at the time. If that is what Mr Bridgen was led to believe, I apologise for my error in judgment in having the conversation.”
Mr Hill was suspended from the Labour Party in September 2019 over the allegations were made but was reinstated in October of that year to fight the general election.
He resigned from his seat in March 2021, prompting the by-election that saw the former red wall seat of Hartlepool swing dramatically to the Conservatives.
Additional reporting by Press Association