Routledge made her Broadway debut in the short-lived 1968 musical Darling of the Day, for which she won a Tony Award as Best Actress in a Musical, sharing the honour with Leslie Uggams of Hallelujah, Baby!. Following Darling of the Day, Routledge had roles in several more unsuccessful Broadway productions including a musical called Love Match, in which she played Queen Victoria; the legendary 1976 Leonard Bernstein flop, 1600 Pennsylvania Avenue, in which she portrayed every First Lady from Abigail Adams to Eleanor Roosevelt; and a 1981 musical called Say Hello to Harvey, based on the Mary Coyle Chase play Harvey, which closed in Toronto before reaching New York.
In 1980, Routledge played Ruth in the Joseph Papp production of The Pirates of Penzance co-starring American actor Kevin Kline and pop vocalist Linda Ronstadt, at the Delacorte Theatre in New York City's Central Park, one of the series of Shakespeare in the Park summer events. The show was a hit and transferred to Broadway the following January, but Estelle Parsons replaced Routledge. A DVD of the Central Park production, with Routledge, was released in October 2002.
She played the role of Nettie Fowler to great acclaim in the 1993 London production of Carousel. In a 2006 Hampstead Theatre production of The Best of Friends, based on a book by Hugh Whitemore, she portrayed Dame Laurentia McLachlan, OSB. The play focused on her friendships with Sir Sydney Cockerell and George Bernard Shaw.
Routledge's early television appearances included roles in Coronation Street and the "Seance in a Wet Rag and Bone Yard" episode of Steptoe and Son (1974). However, she did not come to prominence on television until she featured in monologues written for her by Alan Bennett and Victoria Wood in the 1980s. She firstly appeared in Alan Bennett's A Woman of No Importance in 1982, and then as the opinionated Kitty in Victoria Wood As Seen On TV in 1985. She performed two further monologues in Bennett's Talking Heads in 1987 and 1998.
In 1990, Routledge landed the high-profile role of Hyacinth Bucket in the comedy series Keeping Up Appearances. She portrayed a former working-class woman with social pretensions (insisting her surname be pronounced "bouquet") and visions of grandeur (her often-mentioned but never-seen "candlelight suppers"). She was nominated for two BAFTA TV Awards in 1992 and 1993. The series ended at Routledge's request in 1995.
In 1995, Routledge accepted the lead in another long-running series, the mystery drama Hetty Wainthropp Investigates, which co-starred rising star Dominic Monaghan as her assistant. It first aired in January 1996, and ran until the Autumn of 1998, with one special episode in 1999. She has also played several real-life characters on television including Barbara Pym and Hildegard of Bingen.
In 2001, Routledge starred in Anybody's Nightmare, a fact-based television drama in which she played a piano teacher who served four years in prison for murdering her elderly aunt but was acquitted following a retrial.
Her radio credits include the BBC dramatization of Carole Hayman's Ladies of Letters, in which she and Prunella Scales play elderly women who exchange humorous correspondence over the course of several years.
Patricia Routledge was appointed OBE in 1993, and CBE in 2004.