The Desperate Housewives star bravely recounts the painful experience in her new memoir You Have No Idea, which she co-authored with her mother Helen, and Williams insists it's time she finally went public with her story in a bid to help others.
She writes, "Family friends who had a daughter invited me to visit their friends in California with them. The family we stayed with had two kids. Susan, who was 18, smoked, drove a car, and was the epitome of cool. (My friend) Nancy and I slept in the den. One night Susan crept in. She told me to lie down on the rug. I was confused. Are we going to play a game? As I tried to make sense of it, Susan pulled down the bloomers of my cotton baby-doll pajamas.
"'What are you doing?' I asked. 'Don't worry - it'll feel good.' I lay there paralyzed. What was going on? I didn't speak. She kept at (the molestation) for I don't know how long. She slid my bloomers back up and whispered: 'Don't tell anyone.'
"I climbed into my bed, trying to figure out what had just happened.
"For years I kept Susan's visit to myself. I didn't really understand until college. I was with with my boyfriend and it hit me and I blurted it out: 'Oh my god - I was molested!' After that trip I felt something change in me."
But the Grammy Award-nominated singer, now 49, admits she's in a much better place, telling People magazine, "I don't dwell (on the past)."