As a tertiary, Catherine lived at home rather than in a convent, and she practiced austerities there which a prioress would probably not have permitted. She is especially famous for fasting and practicing physical mortifications that remind modern readers of the extremes of anorexia nervosa, so much so that the recent scholar Rudolph Bell uses her life as an example in his book Holy Anorexia.
In about 1366 she experienced what she described in her letters as a 'Mystical Marriage' with Jesus Christ, and in 1370 she received a series of visions of Hell, Purgatory, and Heaven after which she heard a command to leave her withdrawn life and enter the public life of the world. She began to write letters to men and women in authority, especially begging for peace between the republics and principalities of Italy and for the return of the papacy from Avignon to Rome. She had a detailed correspondence with Pope Gregory XI, also asking him to reform the clergy and the administration of the Papal States. In June of 1376 she went to Avignon herself as ambassador of Florence to make peace with the Papal States, but was unsuccessful. She impressed the Pope so much, however, that he returned his administration to Rome in January of 1377. In the Great Schism of 1378 she was an adherent of Pope Urban VI, who summoned her to Rome where she lived until her death in 1380.
Pope Pius II canonized Catherine in 1461. Her letters are considered one of the great works of early Tuscan literature. Her feast day is 29 April.