At the time, Mantez had no acting experience, but her work on the film resulted in a succession of parts in television series such as Shadow Squad and The Avengers.

Mantez's acting career was boosted with a role as a nurse in the film Life for Ruth (1962), co-starring Patrick McGoohan.

During the 1960s, Mantez appeared in a number of stage musicals; these included Cowardy Custard (the Mermaid Theatre's tribute to Noël Coward), Monty Norman's The Perils of Scobie Prilt, and the West End production of Every Other Evening (starring Margaret Lockwood).

She may be best known for her portrayal of Lieutenant Nina Barry (frequently sporting a purple wig, as did all female staff on Moonbase) in the science-fiction TV series UFO, broadcast in 1970 and 1971.

Her final acting role was as Indigo Jones, a mixed-race lady of means, in an episode of the BBC series The Onedin Line ("Fetch and Carry", 1972).