The Hollywood power couple's marriage came under intense spotlight at last year's Academy Awards, when Smith struck comedian Chris Rock on stage for making a joke about his wife.
But the pair had already been living "totally separate lives" for years before the notorious incident, Pinkett Smith told NBC News.
"By the time we got to 2016 we were just exhausted with trying," she said, in an advance clip from an interview to promote her new memoir, Worthy.
Rumors of affairs and an open marriage were repeatedly denied by the couple for many years.
But in 2020, Pinkett Smith openly discussed an extramarital "entanglement" she had with singer August Alsina some years earlier, during a period of separation from her husband.
According to Wednesday's snippet, that separation has endured up until the present day, although the pair remain legally married.
Asked why the couple had chosen to conceal their separation, Pinkett Smith said they were "just not... ready yet" and "still trying to figure out between the two of us how to be in partnership."
"We just got deep love for each other and we are going to figure out what that looks like for us," Pinkett Smith told People magazine, in a separate interview.
Pinkett Smith also discussed the Oscars ceremony of March 2022, in an excerpt from her memoir published Wednesday.
Rock was presenting on stage when he made a crack about Pinkett Smith's closely cropped head. She has alopecia.
Smith mounted the stage and slapped the comedian hard across the face, before returning to his seat and yelling obscenities at the presenter.
In the excerpt, Pinkett Smith recalls that she initially thought the slap was a "skit," and even once she realized it wasn't, remained "unclear on the reason why Will is so upset."
"We had been living separate lives and were there as family, not as husband and wife," she writes.
Pinkett Smith told People that Rock had once asked her out when he thought she and Smith had separated.
"I think every summer all the reports would come out that me and Will were getting a divorce, and this particular summer, Chris, he thought that we were getting a divorce," she told the magazine.
"So he called me and basically he was like, 'I’d love to take you out,'" she said, adding that she had put him straight that there was no divorce in the offing.
"He was appalled. And he profusely apologized and that was that," she said.
Smith and Pinkett Smith first met on the set of his sitcom The Fresh Prince of Bel-Air in 1994.
Smith was married at the time to his first wife, whom he later divorced.
The couple have two children together, Jaden and Willow. (AFP)