The actress and producer says "the fact that I stood up for myself" was transformative.
"I was keeping my mother abreast of this right towards the end, because she loved asking about it and wanted to know", the 38-year-old continued.
"I was very lucky that the show started up at that time so I could just throw myself into it and distract myself", she said.
While Witherspoon doesn't name her abuser, the actress talks candidly about how it affected her deeply even at a tender age.
"I knew it was going to be very hard, but I just couldn't go any further", the 41-year-old actress responds when asked about making the choice to leave the relationship, adding that she was very young at the time. It's wrought with self-doubt, particularly if someone damages your self-esteem.
"I was a different person too". "People say to me that knew me then, "you're a completely different person", she tells Oprah. And it's part of the reason I can stand up and say, 'Yes, I'm ambitious, ' because someone tried to take that from me".
Despite her red carpet, picture-perfect appearance, it's clear that Witherspoon is able to connect to survivors in ways only an abuse survivor can. The word ambition, she noted, is not seen as being synonymous with being feminine, and that, she added, should change.
Witherspoon, Winfrey and Kaling also cover the March issue of O magazine, on newsstands February 13, and the Queen of Talk speaks on the anti-harassment movement Me Too.
Last October, as dozens of actresses came forward with allegations of sexual misconduct, Witherspoon, 41, spoke at ELLE's Women in Hollywood event about being assaulted as a teenager by a director.
"The understanding was that remaining silent was the condition of our employment".