He served with distinction under King Gustavus Adolphus, who regarded him as the best general in the Swedish army after Lennart Torstenson.

He took part in the Polish-Swedish Wars, in which he first participated in the siege and capture of Riga 1621, and later served with distinction in Poland (1626–1627) and assisted in the defence of Stralsund in 1628.

In 1630 he accompanied Gustavus into Germany, in the Thirty Years' War and in 1631 was appointed colonel of "the yellow regiment," the king's world-renowned life-guards, at the head of which he captured the castle of Würzburg on 8 October 1631.

At the very beginning of the fight he received a gunshot wound to his left knee so severe that he had to retire from the battle.

After the fighting ended Brahe travelled to Naumburg to recuperate but his condition deteriorated over the next few weeks and he finally died on 21 November 1632, at the age of 28.