Born in Marylebone, London, versatile character actress Rosalind Marie Knight was born to theatrical parentage. Her father was the accomplished thespian Esmond Knight. Her mother, the comedienne Frances Clare, often featured in Ivor Novello operettas. Rosalind's interest in theatre was first kindled at the age of six when she and her mother attended a staging of Novello's "The Dancing Years" at Drury Lane. Rosalind was evacuated to the countryside with her nanny during the war years. In 1949, she accompanied her father to the Old Vic Theatre and became enthralled by a production of "The Snow Queen", primarily performed by drama school novices. The following year she won an audition and spent two years at the Old Vic Theatre School. This was succeeded by a lengthy apprenticeship in repertory which led to her gaining further experience as assistant stage manager for the West of England Theatre Company, the Midland Theatre Company in Coventry and the Piccolo Theatre Company in Manchester.

In 1955, she made her first impact on screen as a lady-in-waiting in Laurence Olivier's Richard III (1955), which also featured her father in the cast. A year later, having come to the attention of a movie producer, she played Annabel, one of the schoolgirls, in Blue Murder at St. Trinian's (1957) (decades later, she would return as a teacher in the sequel The Wildcats of St. Trinian's (1980)). This set the tone for a number of subsequent comedic roles which included a couple of early Carry On's and the Tony Richardson-directed Tom Jones (1963), in which she played the giddy Mrs. Harriet Fitzpatrick. While doing the Carry On films she was not under any form of contract and was paid a mere $50 a week. In 1957, Rosalind joined her father in an early BBC adaptation of Nicholas Nickleby (1957) as the spiteful Fanny Squeers. In a later miniseries based on Charles Dickens, Martin Chuzzlewit (1964), she was a splendidly shrewish Charity Pecksniff.