He was born in Trujillo, in the province of Extremadura, the son of Gutiérrez de Sotomayor e Hinojosa and Beatriz de Valmediano. At the age of 15 he joined the army, serving first in Italy until 1567, and then moving to Flanders.
In 1580 he was called back to Madrid by his official duties. King Philip II, seeing his military record, awarded him a knighthood in the Order of Santiago and sent him on a campaign against Portugal. However, at that time news arrived from Chile, where the Arauco War continued and reinforcements were needed. The king subsequently decided to name Sotomayor governor of the district and send him there with a large contingent of soldiers to resolve the situation.