See J. J. Norwich, The Other Conquest (1967).
Malaterra, who compares Robert Guiscard and his brother to "Joseph and Benjamin of old," says of Roger: "He was a youth of the greatest beauty, of lofty stature, of graceful shape, most eloquent in speech and cool in counsel. He was far-seeing in arranging all his actions, pleasant and merry all with men; strong and brave, and furious in battle." Roger shared the conquest of Calabria with Robert, and in a treaty of 1062 the brothers in dividing the conquest apparently made a kind of "condominium" by which either was to have half of every castle and town in Calabria.
Robert now resolved to employ Roger's genius in reducing Sicily, which contained, besides the Muslims, numerous Greek Christians subject to Arab princes who had become all but independent of the sultan of Tunis. In May 1061 the brothers crossed from Reggio and captured Messina. After Palermo had been taken in January 1072, Robert Guiscard, as suzerain, invested Roger as Count of Sicily, but he retained Palermo, half of Messina, and the north-east portion (the Val Demone). Not till 1085, however, was Roger able to undertake a systematic crusade.
In March 1086 Syracuse surrendered, and when in February 1091 Noto yielded, the conquest was complete. Much of Robert's success had been due to Roger's support. Similarly, when the leadership of the Hautevilles passed to Roger, he supported his nephew Duke Roger against Bohemund, Capua, and other rebels. In return for his aid against Bohemund and the rebels, the duke surrendered his share in the castles of Calabria to his uncle in 1085, and in 1091 his inheritance in Palermo. Roger's rule in Sicily was more absolute than Robert Guiscard's in Italy. At the enfeoffments of 1072 and 1092 no great undivided fiefs were created, so the mixed Norman, French and Italian vassals all owed their benefices to the count. No feudal revolt of importance therefore troubled Roger.
In 1091 Roger, in order to avoid an attack from North Africa, set sail with a fleet to conquer Malta. His ship reached the island before the rest. On landing, the few defenders the Normans encountered retreated and the following day Roger marched to Mdina. Terms were discussed with the Maltese qadi. It was agreed that the islands would become tributaries of the count himself and that the qadi should continue to administer the islands. With the treaty many Greek and other Christian prisoners were released, who chanted to Roger the Kyrie eleison (Mulej Hniena). He left the islands with many who wished to join him and so many were on his ship that it nearly sunk, according to Goffredo Malaterra. Roger repatriated Malta to Christian Europe.
Politically supreme, the count also became master of the insular church. The Papacy, favouring a prince who had recovered Sicily from Greeks and Muslims, in 1098 granted Roger and his heirs the Apostolic Legateship of the island. Roger created new Latin bishoprics at Syracuse, Girgenti, and elsewhere, nominating the bishops personally, while he turned the archbishopric of Palermo into a Catholic see. Roger practised general toleration towards Arabs and Greeks, allowing to each race the expansion of its own civilization. In the cities, the Muslims, who had generally secured such rights in their terms of surrender, retained their mosques, their kadis, and freedom of trade; in the country, however, they became serfs. Roger drew the mass of his infantry from the Muslims. Saint Anselm, visiting him at the siege of Capua, 1098, found "the brown tents of the Arabs innumerable". Nevertheless, the Latin element began to prevail, as Lombards and other Italians flocked to the island in the wake of the conquest, and the conquest of Sicily proved decisive in the steady decline of Muslim power in the western Mediterranean from this time.
Roger's first marriage took place in 1061, to Judith, daughter of William, Count of Évreux and Hawisa of Échauffour. She died in 1076, leaving all daughters:
In 1077, Roger married a second time, to Eremburga of Mortain, daughter of "William, Count of Mortain" (probably William Warlenc). Their children were: