Borrell was first seen acting as count during the reign of his father Sunyer II in 945 at the consecration of the nunnery church of Sant Pere de les Puelles in Barcelona.

Borrell's military career seems to have been undistinguished—he is recorded as fighting only two battles and seems to have lost both—and it was under his rule that Barcelona was sacked in 985 by the Muslim leader Almanzor.

Before the attacks of the 980s, and discounting a single raid by the Caliph al-Hakam II soon after his succession in 961, he maintained cordial relations with the Muslim rulers of Córdoba and also sent emissaries to the kings of the Franks.

In the following years, Gerbert studied under the direction of Bishop Atto of Vic, some 60 km north of Barcelona, and probably also at the nearby monastery of Santa Maria de Ripoll.

[14] Borrell II continued to issue documents and tour his domains, however, and when he was taken ill in 993 in Castellciutat near la Seu d'Urgell, the will that he made provided for him outliving his executors.