She was imprisoned, initially for seven years, as a young offender for kidnapping and torturing her stepmother's brother, which was extended to a life sentence for the attempted murder of another inmate[3] who, she said, had repeatedly attacked her.

They broke in, armed with knives, kidnapped the step uncle in a stolen van, and tortured him until he was released by police after almost 24 hours.

[11] Baker spent part of her sentence in HM Prison Full Sutton in East Riding of Yorkshire, during the time serial killer Dennis Nilsen was imprisoned there (2003–2018).

[28] A collection of poems and short stories that Baker wrote during her incarceration was independently published as Borstal to Bedlam: Poetry & Prose from the Gutter to the Grave in mid 2023, in support of her during her re-incarceration and prosecution at that time.

[24] She was allowed makeup but not regular oestrogen for feminising hormone therapy until in her desperation to transition, she cut off her own testicles with a razor blade in 2017.

[30] Baker wrote "if constant bullying, comments, sexual harassment and isolation are your idea of a soft option then I'd suggest you haven't really thought things through.

[4] After release, she wrote an article for Vice magazine about her experiences of love and sex as a transgender woman in male prison.

[35] By 2020, Baker set up an advocacy group called The Transprisoner Alliance to deliver support like letters and cosmetics to transgender prisoners.

[2][12] In 2021 she announced plans to contest the Richmond Park Parliament constituency on a platform of prison reform and opposing gentrification.

[6][36] She received Universal Credit, a supplement for low income, and attended many protests, including some to support her partner, a National Health Service nurse and Unite the Union representative.

[36] In January 2023, Baker criticised Prime Minister Rishi Sunak's blocking of the Gender Recognition Reform (Scotland) Bill.

[20] She led a protest at the inaugural event of gender critical feminist and philosopher Kathleen Stock's The Lesbian Project.

[37] At 8 July 2023 London Trans+ Pride Parade, Baker gave a speech to the crowd where she said "if you see a TERF, punch them in the fucking face", a statement that was videotaped and widely distributed.

[43] On 31 August 2023, she was found not guilty by magistrate Tan Ikram, after she said that she was just trying to get on the front page of the Daily Mail, and wished she could take her words back.

[44][45] As of 21 October 2023, she was still imprisoned, now at HM Prison Isle of Wight, and continued to be supported by protesters from the Free Sarah Jane Baker campaign.