Stålarm was "häradshöfding" (chief administrator of a härad) in Nyslötts (Savonlinna) län in 1583. He served in the Royal Swedish navy as a captain, and was promoted admiral of the Finnish navy in 1588. In 1590, at the beginning of the Russo-Swedish War (1590–1595), he was appointed supreme commander in Narva, where he became governor in 1592. In addition, he became "häradshövding" of Sexmäki (Sääksmäki) in 1593 and of Savolax (Savonia) in 1594. In the same year, king Sigismund confirmed Stålarm as his governor in Narva and also appointed him commander of all forces in Finland and Livonia, after he had made Stålarm his envoy to the Russian tsardom already in 1593. Upon the death of "översteamiral" (supreme admiral) Clas (Klaus) Fleming in 1597, he was given supreme command over the naval and land forces in Finland by king Sigismund and became governor of "all Finland." During the War against Sigismund, he supported Sigismund against his adversary duke Charles of Södermanland.