After an education at Marlborough College (1863-4) and Sandhurst (1865-6), Wyllie entered the Durham Light Infantry and arrived in India in 1867. After serving briefly with the 2^nd Gurkha regiment, Wyllie undertook civil and political employment including positions such as Cantonment Magistrate of Nasirabad, Assistant Commissioner in Ajmer-Merwara, and Assistant to the Governor-General's Agent in Baluchistan. Wyllie took part in the Afghan campaign of 1878-80 and was mentioned in the viceroy’s dispatches (Edward Robert Bulwer Lytton, Earl of Lytton). In 1886 he became a Major of the British Army, and in 1892 a Lieutenant-Colonel. During the 1880s and 1890s Wyllie held a number of assistant resident positions throughout India, and in 1898 gained the appointment of agent to the governor-general in central India. In May 1900 he was transferred in the same capacity to Rajputana where he remained until 1901; notable during this last position was his organization of relief efforts to overcome the famine of 1899-1900. 

Upon his return to London he was selected to work as political Aide-de-Camp for Lord George Hamilton, Secretary of State for India; Wyllie retained the position with Hamilton’s two successors. Wyllie was heavily involved in arrangements for the reception of Indian princes in Britain, especially with regards the coronation of Edward VII in 1902. Wyllie also worked with charities and associations related to India. At a reception given by one such association – the National Indian Association – in 1909, Wyllie was shot by Madan Lal Dhingra, an Indian student. A Parsee physician, Dr Cawas Lalcaca, who sought to protect Wyllie, was also killed. Dhingra and his family were known to Wyllie from his time in India, and Dhingra’s family had maintained a correspondence with Wyllie. Dhingra had himself ignored letters from Wyllie suggesting a meeting at India House. Some argue that Dhingra’s preferred target was either the former Secretary of State Lord Morley or the former Viceroy Lord Curzon, given their greater prestige and connection with the construction of repressive policies against revolutionaries in India; Dhingra was known to have been following both men. Wyllie’s presence at events with Indian students, however, made him an easier target. 

The assassination was met with outrage at home and abroad, with public offices closed in Rajputana upon reception of the news. Wyllie’s widow, Lady Katherine Georgina Wyllie (daughter of a member of the Indian Civil Service), was granted a pension by Viscount Morley, then secretary of state. Memorial tablets to Wyllie were established in the crypt of St. Paul’s Cathedral, London, in Rajputana and central India.