In 1959, Phillips wed the tempestuous Irish actor Peter O'Toole and over the course of their 20-year union, the pair frequently collaborated. They starred as illicit lovers in the BBC production "Siwan" (1960) and acted alongside one another in four films, most notably "Becket" (1964), "Goodbye, Mr. Chips" and "Under Milk Wood" (1971). While her feature career lay dormant throughout the 70s, the small screen offered this marvelous character player several excellent roles. Phillips excelled as the suffragette Emmeline Pankhurst in the acclaimed BBC drama "Shoulder to Shoulder" (1974) and won BAFTA Awards for her turns as the matriarch of a Welsh mining family in the series adaptation of "How Green Was My Valley" (BBC, 1975) and as a power-hungry Roman Empress in "I, Claudius". As the 80s dawned, Phillips teamed with Alec Guinness for adaptations of John le Carre's spy novels "Tinker, Tailor, Soldier, Spy" (PBS, 1980) and "Smiley's People" (syndicated, 1982). Simultaneously, she revived her feature career offering nice turns often in material that was subpar (i.e., "Clash of the Titans" 1981). Occasionally, juicier roles, like Madame de Volanges in Milos Forman's "Valmont" (1989) or Mrs. Archer in Martin Scorsese's "The Age of Innocence" (1993) reminded all of what this actress was capable.

In 1980, Phillips undertook her first stage musical role, that of Vera, in a London revival of "Pal Joey" and a new chapter in her career was born. In addition to appearing in classic roles like "Major Barbara" and "Ghosts", she alternated parts in musicals like "Gigi" and "A Little Night Music". Both aspects of her career dovetailed in "Marlene", her superb impersonation of the Teutonic movie legend. The first half of the show was a behind-the-scenes look at preparations for one of the legendary concerts Dietrich gave late in her life, while the second act was a recreation of said concert. When the show opened in 1997 in London, it was a resounding success. A 1999 Broadway staging met a less receptive response, but Phillips justly received a Tony Award nomination for her performance.