Anselm was born in Baggio, near Milan. He supported the Patarenes, an extreme reforming movement, and was therefore sent on a mission to Emperor Henry III by the Archbishop of Milan. In 1056, he was appointed Bishop of Lucca.
In 1061, without having previously been created [[cardinal}}, he was elected pope with the support of Hildebrand (later Pope Gregory VII). Emperor Henry IV had an antipope, Honorius II, elected. Alexander II was generally accepted as the true Pope after the synod of Mantua in 1064.
His Patarene sympathies were reflected in some of his decrees against simony and decrees enforcing clerical celibacy.
The Papacy was strengthened under Alexander II, who insisted on personal attendance in Rome before he would confer the pallium.
Alexander II gave his blessing to William of Normandy's invasion of England (1066).
He died on 21 April 1073, and was buried in San Giovanni in Laterano.