Rey's first major achievement as a mountaineer and guide came in 1877 when he successfully made the first ever ascent of the Aiguille Noire de Peuterey. Thereafter, Mont Blanc became an important venue for his mountaineering exploits, and he had many regular wealthy clients from across Europe, including Elizabeth Hawkins-Whitshed, Paul Güssfeldt: and Prince Luigi Amedeo, Duke of the Abruzzi.