Though the actors were living completely separate lives for seven years, they were not ready to publicly confirm the news before, she confessed to NBC.
By the time they separated she said that they were "exhausted with trying".
The pair still live separately, but do not plan on divorcing.
Pinkett Smith, 52, told NBC she had promised herself that she and Smith, 55, would never get a divorce and said she has not been able to break that promise.
"I think we were both kind of just still stuck in our fantasy of what we thought the other person should be," she said. The interview came ahead of the release of Pinkett Smith's memoir, Worthy, next week.
She said: "I'm going to be by his side but also allow him to have to figure this out for himself."
There was speculation about the couple's marriage in 2020 when the pair went on Pinkett Smith's Facebook show Red Table Talk and discussed Pinkett Smith's "entanglement" with artist August Alsina.
The actors met in 1994 when Pinkett Smith auditioned for Smith's show The Fresh Prince of Bel-Air, and later married in 1997.
They have two children together - Jaden Smith and Willow Smith - along with Trey Smith, Smith's son with his first wife, Sheree Zampino.
The couple made headlines last year when Smith stormed the stage at the Oscars and slapped host Chris Rock, yelling "keep my wife's name out of your [expletive] mouth".
The incident came after Rock made a joke about Pinkett Smith being bald. The actress suffers from alopecia, a condition which causes hair loss.
In a separate interview, Pinkett Smith told People she initially thought it was part of a planned joke.
"It wasn't until Will started to walk back to his chair that I even realised it wasn't a skit."