The world knows Mariska Hargitay as Olivia Benson, the fiery heroine in 20-plus seasons of “Law & Order: Special Victims Unit.”

That changes Friday as Hargitay’s extraordinary biographical documentary debut “My Mom Jayne” reveals a filmmaker of the first order.

“My Mom Jayne” follows a daughter’s search for her roots and an understanding of Jayne Mansfield, the mother she never knew.

What emerges is a complicated familial history that was played out in international headlines decades ago and, thanks to Hargitay’s tenacity, is pursued, examined and explained today.

Hargitay’s mom was the Fifties’ sex symbol Jayne Mansfield whose fame came as a more overt version of the era’s biggest star, bosomy, blonde bombshell Marilyn Monroe.

Mansfield — born Vera Jayne Palmer in 1933 in Bryn Mawr, PA — grew up determined to be a star. Married and immediately a mother with her first husband Paul Mansfield, she left him behind in Dallas for a Hollywood career.

Jayne Mansfield’s career was launched with the satirical comedy, “Will Success Spoil Rock Hunter?” a smash on Broadway and then a 1957 movie where she triumphed as a one-note parody of Monroe.

Mansfield married Hungarian body builder Mickey Hargitay. They became a celebrity couple famous for their heart-shaped Beverly Hills pool. They opened their lives — and soon all their kids’ lives — to the public.

Mariska Hargitay was three when in 1967 Jayne, just 34, was killed along with two others in a horrific car crash. Mariska and her two older brothers Mickey, Jr. and Zoltan were sleeping in the back seat and survived.

As Hargitay and her brothers each discuss their memories of the accident and its aftermath, breaking down into tears and regret, “My Mom Jayne” clearly shows how PTSD never really goes away but remains, an essential element in living your life.

As director, Hargitay’s biggest revelation is how, in her 20s, she discovered that her father was not Hargitay, the man who loved and raised her and whom she calls Dad, but Nelson Sardelli, an Italian singer Jayne met in Atlantic City and, briefly, lived with in Europe.

Pregnant, Mansfield reunited with Mickey, had Mariska, and then they separated for the final time.

Mariska Hargitay’s journey with her mother is, thanks to the documentary, transformational. As her career collapsed, Jayne Mansfield — an accomplished musician on the violin and piano, a woman who spoke five languages — had issues with drugs, alcohol and men.

Growing up Mariska found little in her mother’s nude Playboy spreads and sexy club act to admire, much less embrace.

That’s changed, of course. Today she can marvel at a celebrity who with five children and a career was both breadwinner and a devoted parent.

“My Mom Jayne” premieres Friday on HBO and streams on MAX.