Bonnie Bartlett Details Past Open Marriage with 'Boy Meets World' Star William Daniels

The celebrated TV actress is opening up about her 72-year marriage in a new memoir.

Celebrated TV actress Bonnie Bartlett is opening up about her life and her 72-year-long marriage to Boy Meets World star William Daniels in her candid and revealing new memoir, Middle of the Rainbow.

In her new book, Bartlett -- a two-time Emmy Award winner for her work on St. Elsewhere in the mid-1980s -- reflects on her childhood and getting into acting in the mid-1950s, as well as her marriage to Williams.

The pair tied the knot in 1951, and while they've maintained a healthy relationship for over seven decades, Bartlett recounted how things were different and somewhat more tumultuous in the early years.

"I guess it was a little bit of an open marriage at first," Bartlett recalls in her memoir, "but that was very painful. That didn’t work well."

"It was a time when people were doing that. It was at a time in New York when there was a lot of sex and a lot of people doing all kinds of things, you know – very free," she continues. "But I don’t know if there was a lack of commitment a little bit, and that’s not good."

For Bartlett, the situation was unpleasant, because "there was a lot of pain connected with any transgression, with any extramarital thing... It was very painful for the both of us."

However, Bartlett says that, for the marriage to have worked for as long as it has, it was "something we had to go through, because we never went through it."

"When we got together I was 18. Bill was my first boyfriend," she writes. "We just had to go through all that and still, we loved each other very much and always have."

The openness of the marriage was something that both she and Daniels seemed to embrace for a while -- although without labeling their relationship as "open," in those terms. Bartlett writes that, in the late 1950s, she had an affair that "lasted a few months" with another actor, but didn't want to leave her husband. "I never felt guilty because I never felt tied to fidelity," she writes, "and neither did Bill."

However, when Daniels stepped out of the marriage with a woman in the 1970s, it left Bartlett emotionally wrecked, and her mindset on fidelity changed. She says she "could no longer tolerate any kind of open marriage."

Through all the ups and downs and storms weathered, the pair have managed to prioritize their love and keep moving forward, together. "We’ve been happy together and sad together, and somehow stayed together for seven decades," she writes.

ET spoke with both Bartlett and Daniels at their home in October, and they opened up about their illustrious careers and lengthy union.

Like Bartlett, Daniels also starred on St. Elsewhere as Dr. Mark Craig -- a role that earned him two Emmys, including in 1986 for Lead Actor in a Drama Series. Bartlett also won an Emmy that year for Best Supporting Actress, as they became the second married couple in history to win an Emmy on the same night for St. Elsewhere.

The pair opened up about Daniels' iconic role as Mr. Feeny on Boy Meets World -- which he very nearly turned down -- and the secret to the longevity of their marriage.

"Lack of imagination," Daniels joked. "[In that] I can't imagine being with anybody else but her."

Middle of the Rainbow -- which details the highs and lows of her career in Hollywood, her marriage and her life on and off set -- is available now.