At a party in London, hosted by Audrey's mother in July 1953 during the British opening of Roman Holiday, Audrey's co-star Gregory Peck introduced her to his friend, American actor Mel Ferrer. Cecil Beaton remembered Mel saying to him that night that Audrey was the biggest thing to come down the turnpike. An immediate attraction developed, despite Mel being almost twelve years Audrey's senior and currently on his third marriage. A besotted Mel, keen to find a joint project, sent Audrey the script for Jean Giaradoux's 1938 play Ondine, a medieval love story of a water sprite and a knight errant and the folly that results from their marriage. Audrey immediately agreed to play the heroine to his knight after filming finished on Sabrina, securing a package deal for both herself and Mel with the Playwrights Company, to be directed by Alfred Lunt.

Audrey and Mel became inseperable during rehearsals in New York, provoking rumours of an off-stage romance. Ondine opened on 18 February, 1954, to rapturous applause. The critics were unanimous in their praise of Audrey's enchantingly lovely performance as the titular water nymph, appearing on stage in a skimpy fishnet bodysuit, which according to Alexander Walker gave the appearance she was naked except for a few strategically positioned wisps of seaweed.Within a month, Audrey had won a Tony award for her performance in Ondine, then only the second woman to win an Oscar and Tony award for best actress in the same year, and became the most highly paid actress on Broadway. However, it would turn out to be her last ever stage appearance - Audrey had exhausted herself, and by June she was advised by her doctor to pull out of the play early and retreat to the Swiss Alps, which would become her lifelong sanctuary. By September, Audrey and Mel Ferrer were married.