Free at last!” whoops Mackenzie Phillips. “Now I can do what I want when I want.” That includes buying her own $150,000 bachelor girl pad atop Laurel Canyon. Her 18th birthday present to herself when all that liberation began (Nov. 10, 1977) was a shocking-red Mercedes 450 SL convertible. And what other high school senior took a vacation to Tahiti after wrapping her third TV season? Sure, the CBS series that bought that freedom is titled One Day at a Time and preaches a more patient and prudent coming-of-age. But Mackenzie has been edging toward the fast lane virtually from kindergarten, when she was tagging along with the Mamas and the Papas, the formative ’60s folk-rockers founded by her dad, John Phillips.

Though her protective Aunt Rosemary Throckmorton (John’s sister), with whom she lived more recently, long forbade dating, Mackenzie is now a girl-about-town with a married man almost twice her age. He’s Peter Asher, 33, the Grammy-winning superproducer (Linda Ronstadt, James Taylor), who is separated from his wife, Betsy. Another sign of Mackenzie’s rebellious passage is that she’s already had a drug bust. An early wine drinker and heavy smoker since 14, she was found last November semiconscious on a West Hollywood street and was arrested for disorderly conduct under the influence of drugs or alcohol. (The DA said they also found a small trace of cocaine in a straw.) It’s a bum rap, by Mackenzie’s account. At a party “Some guy came up to me and said, ‘Here, take this. It’ll make you feel better.’ Like a stupid, dumb shit, I took it. It was a Quaalude. I’d never taken one before. My knees were like jelly. I asked the guy to take me to a coffeeshop, but I fell over just when a deputy came by. So they took me in.” She’s on six months’ probation requiring her to see the same psychotherapist she’d been seeing eight months anyway. “It’s called diversion therapy,” Mackenzie says. “And I’m happily diverted.”

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That Mackenzie is grappling with her problems is heartening enough after her dislocated showbiz upbringing. The long-term influence of Papa John clearly overshadowed that of her mother, Baltimore socialite and ex-ballerina Susan Adams, now remarried to a pool manufacturer. At 5 Mackenzie, temporarily in John’s custody, slept in her own pup tent on a Virgin Island beach with the Mamas and Papas. Her stepmother then, and still a confidante, is the mercurial actress-singer Michelle (Valentino) Phillips.

Mackenzie had determined on a showbiz career by the time she was 10, and two years later with three boys from school, she organized a singing group and played L.A.’s Troubadour on an amateur night. There she was caught by a talent scout for producer-director Francis Ford Coppola, who got her cast as the endearingly out-of-it string bean in George Lucas’ American Graffiti. (She’s now 5’8″, 110 pounds.) Afterward, Papa John protected Mackenzie from star-tripping by packing her off to summer school in Switzerland. In her early teens, he remembers, “she was the wildest kid on earth.” (It was at 12 that she had that star tattooed on her left wrist.)

If her TV role of big sister Julie may seem a little backward, Mackenzie isn’t griping. Her CBS mom, Bonnie Franklin, calls her a “terrific” pro to work with, but the producers hear from Mackenzie when she thinks the scripts are out of character, “because they come up with some pretty weird things that I just wouldn’t say.” Her main contact with her own age group is the Hollywood Professional School, which she attends 8:30 to 12:30 weekdays with One Day co-star Valerie Bertinelli. “We raise hell,” says Phillips, “because it’s such a bore and really an ugly school.” Aside from her twice-weekly maid, Mackenzie now shares her house with her cat, Brains, and two boa constrictors left behind when her big brother Jeff Phillips, 20, moved out to become a recording engineer. “He’s so gross,” Mackenzie wails of Jeff’s habit of naming the live rats he feeds to his reptiles. “He’ll say, ‘Goodbye, Samantha, it was nice knowing you,’ and drop the rat down the snake’s throat.”