After entertaining British troops alongside fellow child stars Julie Andrews and Anthony Newley, Clark made her film debut with A Medal for the General in 1944. By the dawn of the '50s she was a superstar throughout the U.K., with a résumé of close to two-dozen films; 1954's "The Little Shoemaker" was her first Top 20 single, while 1960's "Sailor" was her first chart-topper. Still, Clark struggled with her inability to shed her adolescent image. After selling over a million copies of 1961's "Romeo," she married and relocated to France, establishing a strong fan base there on the strength of hits including "Ya-Ya Twist," "Chariot," and "Monsieur," which spotlighted a new, more sophisticated pop sound anchored by her crystalline vocals.

Riding the wave of the British Invasion, Clark was finally able to penetrate the U.S. market in 1964 with the Grammy-winning "Downtown," the first single by a British woman ever to reach number one on the American pop charts. It was also the first in a series of American Top Ten hits (most written and arranged by Tony Hatch) which also included 1965's "I Know a Place" and 1966's "I Couldn't Live Without Your Love," and the number one smash "My Love." At the same time, she remained a huge star throughout Europe, topping the British charts in 1967 with "This Is My Song," taken from the film A Countess from Hong Kong. In addition to hosting her own BBC series, she also starred in the 1968 NBC television special Petula, which triggered controversy when sponsors requested that a segment with guest Harry Belafonte be cut in deference to Southern affiliates; ultimately, the show aired in its intended form.

As the '60s drew to a close, Clark's commercial stature slipped, although singles like "Don't Sleep on the Subway," "The Other Man's Grass Is Always Greener," and "Kiss Me Goodbye" still charted on both sides of the Atlantic. In 1968, she revived her film career by starring in Finian's Rainbow, followed a year later by Goodbye, Mr. Chips. In later years, Clark focused primarily on international touring, headlining the 1981 London revival of Rodgers & Hammerstein's The Sound of Music; after starring in the 1990 musical Someone Like You, which she also co-wrote, she made her Broadway debut in Blood Brothers in 1993. Additionally, in 1988, an acid-house remix of "Downtown" reached the U.K. Top Ten, another honor for the female singer awarded the most gold records in British pop history.