Clare was born in Assisi, Italy in 1193 as the eldest daughter of Favorino Scifi, Count of Sasso-Rosso. In 1210, Clare heard Francis preaching in the streets of Assisi about his new mendicant order (then newly-approved by the Pope) and was moved by his words. On March 20, 1212, Clare and her sister Agnes fled their home to follow Francis: she and her sister cut their hair short and put on rough tunics to indicate their acceptance of the vows of poverty and celibacy, and stayed with a nearby convent of Benedictine nuns until moving to San Damiano, where Clare and Agnes founded the Order of Poor Ladies (also then known as the Order of San Damiano).
In 1216, Clare accepted the role of abbess at San Damiano, and defended her order from the attempts of prelates to impose a rule on them that more closely resembled the Rule of St Benedict than Francis's stricter vows. Clare also played a significant role in encouraging and aiding Francis, whom she saw as a spiritual father figure: she took care of him during his illnesses at the end of his life, until his death in 1230.
After Francis's death, Clare continued to promote the growth of her order, writing letters to abbesses in other parts of Europe and thwarting every attempt by each successive Pope to impose a rule on her order of women beyond the order she had originally devised, despite the fact that, after 1224, Clare was frequently ill and unable to leave San Damiano.
On August 9, 1253, the Papal bull Solet annure confirmed that Clare's Rule would serve as the governing rule for the Order of Poor Ladies. Two days later, on August 11, 1253, Clare passed away at the age of 60.
On August 15, 1255, Pope Alexander IV canonized Clare as St Clare of Assisi. In 1263, Pope Urban IV officially changed the name of the Order of Poor Ladies to the "Order of St Clare".