Thea King was born in Hitchin, Hertfordshire. She was the daughter of Henry Walter Mayer King, the manager of a family engineering business, George. W. King Ltd., which was based in Hitchin then Stevenage, Hertfordshire, and his wife, Dorothea (nee Hass). She was educated at Bedford High School and won a scholarship to the Royal College of Music where she studied the piano with Arthur Alexander and the clarinet with Frederick Thurston. In January 1953 she married Frederick Thurston but he died from lung cancer in December of the same year. She did not remarry.
She worked as soloist, chamber musician and as a teacher but was probably associated most closely with the English Chamber Orchestra as principal clarinet from 1964 to 1999. She also worked with the London Mozart Players, succeeding Gervase de Peyer as principal clarinettist, the Sadler's Wells Opera Orchestra, the Melos Ensemble and the Allegri String Quartet. She was a founder member in 1953 of the Portia Wind Ensemble, an all female group. She was also a member of the Vesuvius and Robles Ensembles.
Thea King made a special study of lesser known works of the 18th and 19th centuries, especially those of Crusell. She commissioned Elizabeth Maconchy's Fantasia and Howard Blake's Clarinet Concerto. Compositions dedicated to her by British composers include Benjamin Frankel's Clarinet Quintet and Gordon Jacob's Mini Concerto.
From 1961-87, King was Professor of Clarinet at the Royal College of Music and she was a Professor at the Guildhall School of Music and Drama from 1988 until her death. She was a Fellow of both institutions. Thea King was made an OBE in 1985 and was appointed a DBE in 2001.