Horn-Smith joined Vodafone at its foundation in 1984 and held a number of senior posts including chief operating officer, managing director Vodafone International and deputy group CEO. Considered a principal architect in the development of Vodafone's global strategy, during his career with the company he was closely involved in many of Vodafone's major international transactions, including the formation of Vodafone AirTouch in 1999. This merger (of the UK and US businesses) created one of the top 25 companies in the world by market capitalisation. The subsequent acquisition of Mannesmann in 2000, almost doubled the size of Vodafone making it the world's largest mobile telecommunications company and the largest FTSE100 quoted stock. In 1996 he was appointed to the board and in 2005 became deputy chief executive. Horn-Smith was instrumental in the foundation of many of Vodafone's largest subsidiary businesses today (such as those in Turkey, Ireland, Netherlands, South Africa, and Egypt.