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.
During her time with Boston Ballet, Cossar has performed in several of the Company's international tours, including the 2010 Spain Tour, the 2010 Ottawa Tour, the 2012 Helsinki Tour, and the 2013 London Tour.
Cossar's repertoire includes Mikko Nissinen's The Nutcracker and Swan Lake, Florence Clerc's La Bayadère and George Balanchine's complete evening of Jewels, Jiří Kylián's Bella Figura, Peter Martin's Barber Violin Concerto and the world premieres of Jorma Elo's Sharper Side of Dark, Awake Only, and Brake the Eyes.
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.