Born on August 16, 1919,[1][2][3] in Boston, Massachusetts, Thayer was the daughter of silent screen actress Louise Gibney and George Casey.

She began taking dance classes by age 5, and she attended Immaculate Heart College in Los Angeles.

[8] She is most likely to be remembered for her role in the iconic 1970 film Five Easy Pieces as the waitress who refuses to allow Jack Nicholson's character to order a side of wheat toast.

On January 2, 1960, in season 3, episode 16 "The Prophet" of Have Gun - Will Travel, Thayer was cast as Serafina, wife of Colonel Benjamin Nunez (Shepperd Studrick).

[10] After battling Alzheimer's disease for five years, Thayer died at the Motion Picture and Television Fund Retirement Home in Woodland Hills, California, aged 85.