Things became hot and heavy while McCormick and Williams were filming episodes in Hawaii.
"We couldn't hold back any longer," she writes in the book published by William Morrow. "It was our first kiss, and it was long, passionate and deep. It was wonderful, too, though as we continued to kiss and press against each other so closely that we could feel each other's body heat, a part of me - a tiny part, admittedly - said to myself, 'Oh my God! I'm kissing my brother. What am I doing?"'
Now 52, McCormick - who endured a battle with drug addiction and depression - also discusses her dates with Martin and Jackson. It was several years after "The Brady Bunch" ended that McCormick went out with Martin, who had asked for the actress' phone number through friend Chevy Chase.
"I remember him being a very good kisser," McCormick writes about Martin. "But I was insecure and either high or spaced out (most likely both), and I didn't laugh at his jokes.
"Though Steve was too polite and confident of his talent to say anything, I'm sure my inability to carry on a normal conversation or respond intelligently put him off," she writes. "We never spoke again after that date. I've always regretted my behavior because he impressed me as an extraordinary guy. I would've enjoyed a second date."
McCormick's time with Jackson was innocent. They met while she was starring on "Brady" and he was part of the Jackson 5.
"Once we went ice-skating and he held my hand as we glided around the rink," the book recalls. "I wondered if he might try to kiss me, but he didn't. After another outing, he did give me a kiss goodbye. But it was only a gentle peck on the cheek."
There were more men in McCormick's life, which spiraled downward into substance abuse and depression as she struggled to reconcile her Marcia Brady image of the girl next door with her private pain.
In the book, hitting stores Tuesday, she discusses cocaine binges and parties at the Playboy Mansion and the home of Sammy Davis Jr., an unwanted pregnancy and trading sex for drugs.
McCormick was 14 when "The Brady Bunch" debuted on ABC, running from 1969 to 1974. Despite her role as a sunny Miss Perfect, she grappled privately with anxiety and insecurity, the youngest of four children born to a mercurial father who abused and cheated on their mother.
"As a teenager, I had no idea that few people are everything they present to the outside world," she writes. "Yet there I was, hiding the reality of my life behind the unreal perfection of Marcia Brady. ... No one suspected the fear that gnawed at me even as I lent my voice to the chorus of Bradys singing, `It's a Sunshine Day."'
When the series had its final bow, she took up a hard-partying lifestyle in Hollywood, using drugs including cocaine and Quaaludes. She struggled to regain her earlier success, landing some TV and movie roles, but developed a reputation for unreliability due to her addiction, even botching an interview with Steven Spielberg for a role in "Raiders of the Lost Ark" because she was high.
After interventions, stints in rehab and experimental therapies, McCormick began getting sober in 1985 when she married actor Michael Cummings, with whom she has a daughter, Natalie. She continued to fight depression through therapy, medication and the help of "Brady" cast mates.
McCormick, who is also a singer, starred on the Country Music Television reality series "Gone Country" and "Outsider's Inn."
She also confronted her weight issues several years ago as the winning contestant on the VH1 reality show "Celebrity Fit Club."
As for her iconic role, "I'll always be struck by how much a part of people's lives Marcia is and always will be. But now I'm not bothered by the connection. It took most of my life, countless mistakes and decades of pain and suffering to reach this point of equanimity and acceptance," she says.
