Eons before Zooey Deschanel batted her big eyes on “New Girl” (and a few years before the “Mary Tyler Moore Show”), actress Marlo Thomas became TV’s prototypical working it chick, starring in “That Girl” from 1966 to 1971. Since then, she’s also penned kid’s books (“Free to Be You and Me”) and worked as an activist. She’ll be at the National Portrait Gallery on May 1 for a Smithsonian Associates event to speak about her new book, “It Ain’t Over…Till It’s Over” ($27, Simon & Schuster), which follows women who have reinvented themselves in mid life.

What keeps women from reinventing themselves?
A lot of women in their 40s get stuck, and their dreams run out on them. They’ve raised a family, and they’re seeking what to do next. It’s exciting, but it’s scary.

Why change your life?
It’s such a big thing to do. But sometimes circumstances force it. And women over 40 are not welcome with open arms in the job market. In almost every field, it’s harder to get in if you are older.

What did some of the women in the book do?
One woman, she was a graphic artist, but she’d always wanted to be a doctor. So at 38, she went back to medical school, and she did it. And another woman, who actually is from D.C., started a cupcake truck [Curbside Cupcakes].

Any advice for people looking to make big life changes?
You must do a thorough reassessment of yourself. Ask yourself if you are doing the thing you wanted to do. Think about what would make you happy. Being happy is something to aspire to.

A lot of the women in the book became entrepreneurs. How can other women come up with good ideas for businesses?
Use the resources around you. Who can help you — a friend? Your husband? Then think about something that there’s a need for, say, a mobile manicure and pedicure service. Find out what you like to do, and then take a step every day.

On “That Girl,” you really were one of the first career women on TV.
I didn’t know I was going to become a feminist heroine! But I knew “That Girl” was something new, because it was hard to get it on the air. My agent had sent me so many scripts where the
woman was the daughter or wife of someone, and I liked that it was a show about the girl being the somebody.

What heroines have you liked on TV in decades since?
“Kate & Allie” — it was pioneering — they were single moms. And “Friends” and “Girls” both changed things.