Julia Mulligan, who has overseen policing in North Yorkshire since 2012, writes in The Yorkshire Post today that she "can't keep this secret any more" and wants other victims to know she is "by their side".
The 51-year-old, who grew up in a North Yorkshire village, said the sexual assault took place 36 years ago and she only told her family and closest friends about it late last year.
Mrs Mulligan says she did not report the incident to police at the time as she was "dealing with the here and now of what's happened" and partly blamed herself for her ordeal.
She does not want to speak publicly about the specifics as this might prompt the police to try and investigate the incident, adding: "I have lived with this for so long now that I don't want to go there."
Mrs Mulligan, who last year took over the governance of North Yorkshire's fire and rescue service as well as its police force, said her job brought her into contact with "many women who talk about what’s happened to them, to try and make it better for others".
She wrote: "I have spent time with children who have been exploited, and I know in different circumstances, that could have been me.
"Like many others, my past is material to who I am today, and what I do today. So, when I listen to these people, I feel it might make more of a difference to say that not only am I on their side, but I am also by their side."
The North Yorkshire Police, Fire and Crime Commissioner said she had been influenced by the recent #Metoo campaign to highlight the widespread prevalence of sexual assault and harassment.
But she said part of the reason for her speaking out was the allegations of bullying behaviour made against her by her former staff, which were made public last year and upheld by the North Yorkshire Police and Crime Panel.
She said: "I know I am far from perfect, indeed I am my own harshest critic. After all, I have spent 36 years beating myself up for a situation that I still think today was partly of my own making.
"I hate to ever think I had either intentionally or unintentionally made someone upset or feel unvalued, and for that I am deeply sorry.
"But those characterisations hurt. I can't hide that - and they brought everything back I've hidden away for so long."
Mrs Mulligan says change is not happening fast enough in the way sexism and sexual assaults are dealt with, and criticised a child abuse campaign by her own police force which was removed over claims of victim-blaming.
She said that in North Yorkshire new services had been created to support victims and to help tackle the historically low rate of rape convictions nationwide.
She wrote: "These services are available to children, men and women who have been harmed, and they don’t have to involve the police.
"Had such services been available when I was 15, perhaps things would have been different, but I am yet to be convinced that society has moved on as much as we would like."
She added: “I’ve spent years pretending my assault didn’t happen, boxing it off in my brain. Locking it up, full on blaming myself. No longer. I want to help those who have experienced what I went through.”
Support is available for victims at rapecrisis.org.uk or by calling 0808 802 9999.
A full interview with Julia Mulligan will appear in Saturday's edition of The Yorkshire Post.