Shearer was born in Toronto, Canada, daughter of Constance and John Porter Shearer, who moved to Newark, Wayne County, New York when she was a small child. Her father, a commercial artist, took a position with the Bloomer Bros Company. After graduating in 1930 from Newark High School, she studied at Skidmore College in Saratoga Springs, graduating in 1934. She then pursued modern dance at Bennington College's summer workshops in Vermont, with Doris Humphrey, Martha Graham and Hanya Holm.
Shearer's first solo concert in Manhattan in 1941 at Carnegie Hall, caused a sensation. Shortly after her New York City triumph, she walked away from the fame that was opening for her, settling instead in the American Midwest in the mid-1940s, where she continued to perform in the Chicago area, and inspired numerous students of dance, including John Neumeier who is now director of the Hamburg Ballet.
Dance historian Margaret Lloyd, in The Borzoi Book of Modern Dance, described Shearer as "a perfectionist who likes to believe that perfection is humanly attainable". Shearer was among the first performers to tackle spiritual and social justice issues, such as the plight of factory workers, a theme of one of her pieces. She drew ideas and inspiration from a variety of artistic influences, including lengthy correspondence with choreographer and dancer Agnes de Mille and writer Virginia Woolf.
Shearer depicted both spiritual visions and human foible in her works, which were predominantly solo concerts. She created "Let the Heavens Open That the Earth May Shine" in 1947 which celebrated spiritual ideals. "In a Vacuum" (1941) explored earthly problems and portrayed an assembly-line worker with physically demanding but unrelated movements that suggested dehumanization. She created "Once Upon a Time" in 1951 which was a suite of solos for fantastically named characters. Thus Medmiga was an ominous witch, Yanchi was fey, Relluckus was woebegone and Ziff fluttered aimlessly.
Shearer choreographed group works, among them "Fables and Proverbs" (1961) and "The Reflection in the Puddle Is Mine" (1963).
In a photo book by John Martin, Shearer is often seen wearing loose-fitting garments or highly theatrical costumes. Combining the technique of ballet and the freedom of modern dance, Shearer used a pointed or flexed foot, long extended limbs, and contorted shapes or straight lines of the body.
Many of Shearer's productions were in collaboration with Helen Balfour Morrison, a photographer and filmmaker who documented Shearer's career.
Shearer was appointed artist-in-residence at the Arnold Theatre of the National College of Education (now National Louis University) located in Evanston, Illinois, in 1962. The school was looking to have an artist of great caliber working close by.
As artist-in-residence, Shearer was given the freedom to create works with her company, derived from her repertory, whenever and however she pleased. Her only obligation was to produce one piece that would be performed at the institute's annual assembly. John Martin of The New York Times wrote that Shearer's appointment was the start of alliances formed between established artists and educational institutions.
The Morrison-Shearer Foundation, established in 1991, preserves the works related to the careers of photographer Helen Balfour Morrison and Shearer. The first volume of Shearer's three-part autobiography, Without Wings the Way Is Steep – The Autobiography of Sybil Shearer (a title taken from one of her dances), was released in 2008.
Shearer made her last appearance, at age 93, at the Art Institute of Chicago, in one of her solos called Flame. She died, aged 93, in Northbrook, Illinois following a stroke.
Shearer was celebrated by the poet Gary Forrester in "The Beautiful Daughters of Men: A Novella in Short Verse from Tinakori Hill". (The Legal Studies Forum, Volume XXXIII, Supplement No. 2, West Virginia University (2009), ISSN 0894-5993; a journal established by the American Legal Studies Association.