But in her new memoir, "Here's the Story: Surviving Marcia Brady and Finding My True Voice," the actress writes of her romance with TV sibling Barry Williams, who played Greg Brady, dates with Michael Jackson and Steve Martin, her addictions, depression, and trading sex for drugs.
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.
"I always had this smile on my face," she said in September 2007 in a video released by HarperCollins, "but deep down inside, I felt like I was crying, and very sad."
"Yes, I knew that there were things going on that were not particularly OK," says Williams, "and I empathized with the pressure of having to be this smiling, cheery, wonderful, no problems, no issues kind of image and trying to live up to it."
"When you're a beautiful girl," says TV Guide Senior Correspondent Zoe Alexander, "and you're in Hollywood, and you're swept up in a lot of the drugs sex and rock 'n' roll that this town is full of, you know, it's easy to fall into that. ... But the fact that it was somebody who appeared so together on the screen is what's so shocking about the whole thing."
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 not available for comment on her book.
McCormick was 14 when "The Brady Bunch" debuted on the ABC network, 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."'