Brunner was the son of a wealthy farmer, and inherited land and status. His home, in the hamlet of Hasli, was less than a kilometer from the monastery. In 1363, the monastery was severely damaged by a fire, and Brunner gave them a vineyard located in Thalwil to provide financial support. At some point he joined the order and on 26 April 1380, he was elected abbot. He was the first head of that abbey to come from largely peasant stock. In 1386 troops of the Swiss Confederacy pillaged the monastery during their struggle against the Habsburgs known as the "Sempacherkriegs" (War of the Sempacher, or the War of Lucerne's expansion), which culminated at the Battle of Sempach. Rebuilding the monastery and the local infrastructure was a slow process. In 1399, to compensate for the harm suffered and to help keep their loyalty, Brunner convinced the Habsburg Duke Leopold IV to donate the benefice of the parish of Villmergen to the monastery and again in 1403 the parishes of Sursee and Oberlunkhofen.