[4] Her lyrical technique, her unconventional style, and the very intense dramatic powers were developed through a wide range of roles MacMillan started to make on her regularly[5] including The Girl in The Invitation (1960) and The Fiancée in Le baiser de la fée (1960).

[10] Seymour was prima ballerina at the Berlin Opera Ballet (1966–1969) under MacMillan's direction, where she danced the first performance of his Concerto, whose second movement was inspired by her magnetic plasticity,[11] and created the turbulent role of Anna Anderson in the one-act version of Anastasia (1967).

She worked with different choreographers from John Cranko, Antony Tudor and Jerome Robbins to Glen Tetley, Lar Lubovitch and Roland Petit and was often partnered by her beloved friend Rudolf Nureyev[12] (La Sylphide, Raymonda, Apollo and others).

Ashton created for her a solo called Five Brahms Waltzes in the Manner of Isadora Duncan (1976) and the role of Natalia Petrovna in A Month in the Country (1976, with Anthony Dowell as Beliaev).

She also appeared as an actress in the 1987 Herbert Ross film Dancers with Mikhail Baryshnikov and in Wittgenstein by Derek Jarman (1993), playing the part of Ballets Russes's Lydia Lopokova.

[28] She created a rock dance called Seymour's Circus and came back to the stage in Gillian Lynne's A Simple Man with Northern Ballet Theatre (1987), in Escape at Sea with Second Stride (1993) and with Adventures in Motion Pictures in Matthew Bourne's Swan Lake (1996) and Cinderella (1997).

[29][2] In 1989, at the invitation of Peter Schaufuss of the English National Ballet, Seymour came out of retirement to dance for the first time as Tatiana in Cranko's Onegin in London[30] and again the title role of MacMillan's Anastasia that earned her a rapturous ovation in New York.