The Decision was Musgrave's major work of the 1960's, produced for the first time by the New Opera Company in 1967. A full scale opera, it deals with the grim realities of a mining accident, and confronts a variety of moral questions. It is illustrative of the changes that her compositional style was undergoing at the time, for it is imbued with a distinctive energy. Much of the immediate work that followed was derived from this stylistic transformation. Her concertos also reflect a heightened dramatic sense. They contain asynchronous passages and borrowings in the manner of Charles Ives, a composer whom Musgrave studied and emulated. Musgrave's concertos are complex, dramatic happenings in which the soloist affects different groups of instrumentalists throughout the orchestra. In her Clarinet Concerto, commissioned by the Royal Philharmonic Society, the soloist walks about the stage while interacting with different groups of instrumentalists in the orchestra. In her French horn concerto, the soloist "activates" a group, and then conducts them. The soloists in her concertos initiate interactions among various groups in an abstract dramatic medium in which the orchestra divides and recombines, and eventually unites as a tutti ensemble.
Thea Musgrave's later work has centered on the operatic. Her first major opera of this period is The Voice of Ariadne, a sophisticated tale about being haunted. Mary Queen of Scots (1977) was the first opera for which Musgrave wrote her own libretto. She has continued this practice in works such as A Christmas Carol and Harriet, The Woman called Moses.
Musgrave's compositions have been performed by most of the major symphony orchestras, and she has often conducted her works as well. She has won many awards, including the Koussevitsky award in 1973. The Voice of Ariadne was performed by the New York City Opera Company, and her Clarinet Concerto by the New York Philharmonic. She has lectured at major universities both in the United States and abroad, and in 1987 was made a distinguished professor at the City University of New York.