St Dominic was born about 1170 in Castille, in what is now known as northern Spain. It is said he was a member of the Guzman family and was born in Caleruega, half way between Osma and Aranda, He became a priest and a Canon of the Cathedral at Osma, occupying a position of responsibility in the cathedral chapter.

In 1203 St Dominic accompanied his bishop, Diego, on a diplomatic mission to Denmark. Returning through southern France, they encountered a group of Cathar heretics, and a few Cistercians appointed by the Pope to reconvert the heretics to Christianity. Diego encouraged the papal legates to adopt the lifestyle of the Cathars, a Gospel-poverty which the heretics found more compelling than the comfortable lifestyle of Church leaders. Diego was obliged to return to his diocese, and St Dominic remained in the region, preaching the Gospel with the permission Bishop Fulk of the diocese of Toulouse.

In 1206 St Dominic founded a house for women in Prouilhe, similar to the Cathar houses of the "perfectae"("perfects") into whose care Dominic had seen poor families entrust their daughters. This community served as a shelter for women converted from heresy and a base for Dominic's preaching. The foundation marks the beginnings of the enclosed Dominican nuns.

Devoted to his task and a life of prayer, Dominic attracted followers as he preached throughout southern France, hoping to convert heretics. At the Fourth Lateran Council (1215), he sought papal approval for an "Order of Preachers", but the Council decreed that no new religious order could be established unless it adopted an existing rule. With the agreement of his brethren, St Dominic adopted the Rule of St Augustine. In January 1217, Honorius III recognised Dominic and his followers as "preachers in the territory of Toulouse." The Order of Preachers was born.

In August 1217 St Dominic sent the brethren to Madrid and Paris, and a foundation followed in Bologna. From the beginning, St Dominic recognised the Order's need for preachers with a good formation in theology and philosophy. Dominican priories were founded in university cities across Europe. The second General Chapter of the Order (June 1221), sent a delegation of friars to Oxford, England, the last university lacking Dominicans.

St Dominic's personal dream was to preach in Eastern Europe. It remained unfulfilled. He travelled towards Bologna in 1221, arriving in mid-July, and was soon seriously ill. St Dominic died on 6th August, 1221 in the presence of his brethren, and was buried in the priory church, "under the feet of his brethren."