Munro grew up a bold, powerful and fearless man, playing a conspicuous part in the history and feuds of his time. He entered the army and accompanied his famous uncle, General Robert Monro (and his great-uncle, chief Robert Munro, 18th Baron of Foulis) to the Continental Thirty Years' War, in which he very rapidly distinguished himself. In 1629 when the war broke out between Sweden and Austria, George Munro gave his services to Gustavus Adolphus and served under him with distinction. George Munro commanded the left wing of the Swedish army at the Battle of Lutzen on 6 November 1632 in which the Swedish army was victorious over the Imperialists. However, after Lutzen, arguing ensued amongst many of the officers of the Swedish army and as a result they were defeated at the Battle of Nördlingen. The petty differences on the part of those in command led to no properly defined plan of attack and George Munro was so disgusted with these matters that he returned home to Scotland.