Being taunted with names such as Sloppy Poppy, Poopy Poppy and Floppy Poppy at school did not make for a pleasant experience, so she left at the legal age of 14 years and 9 months. She says that she "was tortured" and she "hated school." "[I had] red hair, freckles, and my name was Poppy Petal. It was like hell. I'm still not over it." She was kicked out of every school she went to in Sydney for such things as eating with her elbows on the table and wearing the wrong underwear. "You had to wear regulation bloomers. We would line up and have random checks. We had little dresses and we'd lift them up." After leaving school she took on a job waitressing at one of her father's restaurants, but claims to have been a horrible waitress and was fired by her father. At one point, Poppy was part of a youth acting class which toured Sydney schools performing Shakespeare's "Twelfth Night". "'No, sir. No jot,' that was my line. You know how people say there are no small roles, only small actors? At one point, I got so bored, I looked at the girl playing the other guard and just lost it onstage. You know when you get the giggles and you try to stop and it gets worse? Afterwards, I was telling myself maybe I'm not cut out for this."

At the age of 16 Poppy left home to travel around Bali with her then boyfriend. Two years later she decided to head for the states, following an American exchange student she was in love with who had been a school friend of her brother's in Sydney. She flew to Florida to pursue him, but the relationship fizzled down within a week and she found herself nearly broke in Sarasota. She had connections in New York and Los Angeles, and when the New York friends weren't home to answer her call, she got on a Greyhound bus and headed for Los Angeles in hopes of an acting career, adopting her mother's maiden name of Montgomery. "The bus driver bought me McDonald's because I was so broke," she says. Staying at her friend's house in Los Angeles--where she would drive around to auditions in a car with no windows or headlights--she sent a head shot nearly every day to Julia Robert's former manager, Bob McGowan, whom she had read about in a book entitled "How to Make it in Hollywood". He said that he didn't "handle unknowns," but Poppy was persistent, and he eventually found some agents for Poppy to meet with who then signed her. Her first gig was in a Carl's Jr. commercial that never aired.

Between early guest appearances on television shows such as "Party of Five" and "NYPD Blue", and small roles in films such as "Devil in a Blue Dress" with Denzel Washington, Poppy worked at The Gap clothing store. Her break came in 1996 when she was cast as a member of the critically acclaimed, yet low-rated television show "Relativity". Also in 1996, Poppy had a starring role in the Sci-Fi TV movie "The Cold Equations" with Bill Campbell. In 1998 she landed her first prominent big screen role in "Dead Man on Campus" with Mark-Paul Gosselaar and Tom Everett Scott. She soon found herself acting alongside veterans such as Diane Keaton and Eddie Murphy in the films "The Other Sister" and "Life", respectively, both in 1999. By 2000 she had completed the critically acclaimed independent feature "This Space Between Us" and the short film "Men Named Milo, Women Named Greta". Also in 2000, she starred in another television show, the short-lived UPN cop series "The Beat". Her last show before finally landing a hit with CBS's "Without a Trace" was the WB's low-rated thriller "Glory Days".

At an early age, Poppy had become fascinated with screen legend Marilyn Monroe. She had posters of her on her bedroom walls, had seen her movies (her favorite is 1961's "The Misfits"), and read biographies on her by the age of 12. In 2001, Poppy landed her dream role and portrayed Marilyn Monroe in the 4 hour television mini-series "Blonde", based on Joyce Carol Oates' fictional novel of the same title. She received rave reviews for her performance. "When I told my mother I got the part of Marilyn Monroe, she didn't even bat an eyelash. She said, 'Well, you've been rehearsing for it your whole life.'" Poppy stopped working out and doing yoga in order to to gain about 12 pounds for the role to obtain the lush figure of Marilyn Monroe. "I was terrified, especially being Australian playing an American, and we were shooting in Australia. My main concern was that it had been done, you know, a lot before this. There's been a lot of Marilyn done in every which way, and I didn't want to do the standard thing, you know, which was sort of the caricature of Marilyn Monroe." For research, she chose not to speak to people who actually knew Marilyn because she says "there's such a myth surrounding Marilyn. Everyone that you talk to [and] the books written about her say things that are polar opposites." Poppy loved playing Marilyn so much that she says she "could have done Marilyn forever."

When questioned as to why she no longer has an Australian accent, Poppy replies "It is much to the mortification of my family that I sound this way. I was on a show called 'Relativity' when I first got here, and I kept thinking I was going to get fired if my Australian accent came through. So, I basically started talking in an American accent all the time. Initially, I could just talk very slowly--enunciate everything--and it sort of became second nature." After working in the states for so long, she says that she "can't do the Australian accent anymore. I went for an audition to play an Australian girl and I thought I was a shoo-in and it was going to be wonderful--I was going to get the job. And the feedback to my manager was it was the worst Australian accent they ever heard. So I think I've kind of beaten it out of my system."