Recent seasons have seen Millar perform with the Berlin Philharmonic, Orchestre National de France, Brussels Philharmonic, Staatskapelle Dresden, New York Philharmonic, São Paulo Symphony, and the London Symphony Orchestra in London and at the Edinburgh International Festival, working with Simon Rattle, Jaap van Zweden, Kazushi Ono, Simone Young, Cristian Măcelaru, and Nicholas Collon. In 2022/23, Millar returned to the LA Phil and San Francisco Symphony to perform Messiaen’s Trois petites liturgies de la présence divine under the baton of Michael Tilson Thomas.

In 2016 Millar premiered the ondes martenot part specially written for her by Thomas Adès in his opera The Exterminating Angel at the Salzburg Festival and subsequently at the Royal Opera House, Covent Garden; Metropolitan Opera; and Royal Danish Opera. The 2016/17 season saw her take part in a 10-concert tour of the Turangalîla-symphonie with the Simón Bolívar Symphony Orchestra and Gustavo Dudamel, beginning in Caracas and culminating in a sold-out concert at Carnegie Hall.

Millar has recorded Turangalîla with the Bergen Philharmonic Orchestra for Juanjo Mena and the Trois petites liturgies with the Seattle Symphony and Ludovic Morlot as well as the London Sinfonietta and Terry Edwards. She has played in well over 100 film and television productions and has written music for film, television, and theater, including scores for Robert Wise, Arthur Penn, Martha Coolidge, and Peter Yates.