Rachel Cossar of Salisbury, England, began her training at the National Ballet School of Canada, Royal Winnipeg Ballet School, École Supérieure de Danse Contemporaire before joining Boston Ballet School as a trainee. Cossar danced with Boston Ballet II for the 2008-2009 and 2009-2010 seasons before she was promoted to company dancer in 2010.
Cossar’s repertoire includes Mikko Nissinen’s The Nutcracker and Swan Lake, John Cranko’s Romeo and Juliet, Florence Leclerc's La Bayadere, George Balanchine's complete evening of Jewels, Jiri Kylian's Bella Figura, Peter Martin's Barber Violin Concerto and Jorma Elo's Brake the Eyes and Sharper Side of Dark.
Cossar has competed in extensive rhythmic gymnastics competitions including the World Championships (Budapest), World Club Championships (Japan), Pan American Championships, Grand Prix (Israel, Russia), and World Gymnaestrada. Cossar was a member of the Canadian National team for ten years and was ranked third when she retired.