Sinclair was the daughter of Mr Flu and Henriette Tyndal, born in Parimaribo, Suriname.
Sinclair began her career at age 18 in 1939. She danced and acted in many Broadway shows and tours, including starring as Ariel in The Tempest on tour (1945), and was featured with Gene Kelly in Pal Joey on Broadway (1940–41). Her Broadway credits include Happy as Larry as Lachesis (1950); As the Girls Go as Miss Swenson (1948–50); Sons o' Fun as Little Nell and Ensemble (1941–43) and Too Many Girls (1939–40).
In 1932, Sinclair signed a contract with Metro-Goldwyn-Mayer.
Sinclair became a regular on the TV variety shows of the 1950s, usually starring with longtime dance partner Ken Spaulding. She was a regular on The Dave Garroway Show from 1953–54, and made a number of guest appearances on The Ed Sullivan Show. Life Magazine pictured her on its cover on January 25, 1954.
She met Sol Tepper, an orchestra musician, while performing in Billy Rose's Aquacade at the 1939 New York World's Fair. They married on July 28, 1942; the couple had one child, a son, Ronald Tepper (born July 29, 1943).
After retiring from performing, she had several successful careers, including casting for television, floral design, and as a travel agent. In her late 60s, she began doing portrait sculpture, both in-the-round and bas-relief, one of which hangs in the lobby of the Players' Club in Manhattan.