Before it, she had not starred in a movie since 1990 when she played the title character in Stanley and Iris alongside Robert De Niro.
The two-time Academy Award winner was quoted in the Daily Mirror saying: "The biggest stroke of luck was getting the role in Monster-in-Law at a point in my career when I had been out of the acting business for 15 years or more.
Jane Fonda has heaped praise on her former Monster-in-Law co-star Jennifer Lopez. Photos / Getty Images
"It was with Jennifer Lopez, and it was a great comeback for me. I was almost 65 years old when I got this script out of the blue.
"It was the only smart career thing I ever did. I thought, 'People are going to come to the movie to see JLo, but they'll rediscover Fonda', and that's what happened."
After the role alongside Lopez, Fonda landed parts including that of Nancy Reagan in The Butler and Grace Hanson in Netflix's Grace and Frankie.
Fonda has also said that she feels more youthful aged 84 than she ever did in her 20s.
She admitted that she felt "lost" when she was younger and was "very unhappy, old and didn't feel like I would live for very long".
But she has said she regrets having a facelift in a battle to maintain her youth.
Fonda admitted she is "not proud" of the operation and wouldn't put herself through it again, telling Vogue: "I had a facelift and I stopped because I don't want to look distorted. I'm not proud of the fact that I had one.
"Now, I don't know if I had it to do over if I would do it. But I did it. I admit it, and then I just say, okay, you can get addicted. Don't keep doing it. A lot of women, I don't know, they're addicted to it."
