Cynthia Millar studied the Ondes Martenot first with John Morton in England and later with Jeanne Loriod. Since Cynthia Millar first performed the Turangalila-Symphonie at the BBC Promenade Concerts in London with Mark Elder and the National Youth Orchestra of Great Britain and she has performed all over the world.

Her work as a performer in over one hundred film and television scores, for composers including Elmer Bernstein, Richard Rodney Bennett, Maurice Jarre, Henry Mancini and Miklos Rozsa, was Cynthia Millar's introduction to the world of film music. She gave the world premiere of Elmer Benstein's Ondine at the Cinema in a special concert to celebrate the composer's 80th birthday with the Royal Philharmonic Orchestra at the Royal Albert Hall, London.

She has also written a number of film scores herself, including the music for Arthur Penn's "The Portrait" starring Gregory Peck and Lauren Bacall; Martha Coolidge's "Three Wishes" and Peter Yates's "The Run of the Country", Jason Freeland's "Brown's Requiem", "A Storm in Summer" directed by Robert Wise, and "Confessions of an Ugly Step Sister" directed by Gavin Millar and staring Jonathan Pryce. For television she composed the music for a major documentary series "Stephen Hawking's Universe" for BBC/WNET.