Law & Order: Svu star Mariska Hargitay was only 3 years old in 1967 when a horrendous car accident took the life of her mother, Jayne Mansfield, as she and two of her siblings slept in the back seat. The chauffeur and Mansfield’s lawyer boyfriend also were killed that night, and Hargitay was found injured under the passenger seat. Now, nearly 60 years later, Hargitay, an Emmy-winning star in her right, is coming to terms with not only losing her mom at such a young age but also getting to discover just who she was, a parent she says she has no memory of ever knowing.

To do this, she has directed and narrates a new HBO Films documentary My Mom Jayne, which explores her very personal and highly emotional journey to meet the woman the public knew well as a blond bombshell movie star of the 1950s and ’60s but also a...