Born December 2, 1795 in Wingham, Kent, Miller was fluent in several languages by the age of seventeen, when he enrolled in the British army to fight in the Napoleonic Wars, taking part in the Siege of Badajoz and Battle of Vittoria under the Duke of Wellington. In September 1817, hearing of the wars in Latin America, he set sail for Buenos Aires to join San Martín's Army of the Andes. He took part in San Martín's liberation of Chile, participating in the decisive battle of Maipú, and then joined Lord Cochrane as the commander of marines in the Chilean Navy. He participated in the Cochrane's Capture of Valdivia leading a force 60 soldiers. After this he participated in the failed expedition to Chiloé and lost the little but significant Battle of Agüi.