EXCLUSIVE: Mariska Hargitay's Untold Story — 'Law & Order' Legend Shares Bombshell Family Secret About Her Late Mother Jayne Mansfield... After Lying for 30 Years

Mariska Hargitay remembers it like it was yesterday.

She was 30, and she saw that Italian entertainer Nelson Sardelli – the man she'd learned five years earlier was her biological father, simply by seeing a photo of him – was playing in Atlantic City, RadarOnline.com can reveal.

So she and a friend went to see his show. Afterward, Hargitay came face-to-face with Sardelli for the first time, and what happened next changed her life forever.

"He looked at me and burst into tears," she recalled. "He told me the whole story. And I've kept it a secret ever since."

The whole story goes like this: When Hargitay's mother, Hollywood bombshell Jayne Mansfield, filed for divorce from Mickey Hargitay in 1963, she began a whirlwind romance with Sardelli, only to reconcile with Mickey several months before Mariska's birth in 1964.

Mansfield's love affair with Sardelli– during which Mariska was conceived – ended abruptly one day when Mansfield asked him a question: Would he take back a woman if she got pregnant by another man?

Sardelli, realizing Mansfield still loved her estranged husband, told her it was over on the spot, and they never saw each other again.

In Mariska's HBO documentary, My Mom Jayne, Sardelli said he never contacted Mansfield, nor did he try to see Mariska out of respect for Mickey, who raised her and said till the day he died at age 80 in 2006 that he was her father, even though he knew Sardelli was her biological father.

All three kept the secret for over 30 years.

Mariska even forced Sardelli's children – her half-siblings – to lie about their true relationship while young because her half-sisters discovered the truth when they found a letter Mariska's grandmother – who also knew the truth – wrote to Sardelli asking him to claim Mariska.

"I never set out to hurt anybody, and I'm sorry if I did," the Law & Order: SVU star told her half-sisters (who look exactly like her) in the documentary.

"That's so hard, that I didn't have the wisdom at the time to say that was not yours to carry. The fact is, I was wrong, because you guys had to live all these years with the secret."

Why did she really lie? To protect her father, Mickey. When Mariska learned the truth at age 25, she confronted Mickey. He vehemently denied it and grew angry.

"I was in so much pain, but I could see his pain was almost worse. So I decided I would never talk about it again, and I never did."

Mariska adored her father.

However, as the years passed, Mariska began to embrace her mother, and she wanted to share her story. Mariska also wanted to tell her own story and make up for past mistakes.

As she said: "Sometimes keeping a secret doesn't honor anyone."

"He was my everything, my idol," she said. "He loved me so much, and I knew it."

Mickey raised the kids on his own after Mansfield was tragically killed in a car accident at age 34 in 1967. Mariska, then 3, was asleep in the back seat of the car when it collided with a tractor trailer. She miraculously survived, along with her siblings, but their mother did not.

"I don't remember the accident," Mariska confides 58 years later. "I don't even remember being told that my mother had died. I look at photos, and I don't really remember anything until I was 5."

Mariska's documentary is getting rave reviews and could win the star an Oscar. But she didn't do it for recognition. She was tired of keeping secrets. She wanted to make everything right. "She's done hiding things," said an insider. "She's an open book. There are no more secrets."

She also wanted to honor her mom. Growing up, she pushed her mother out of her mind. She didn't like Mansfield's image as a blond bimbo. "I was embarrassed by the choices that she made," she shared.