[2] He started his Public Service career as the Supplemental Clerk at the Privy Council, and eventually became a minor Conservative member of the British Parliament, representing King's County from 1859 to 1865.

Although Hennessy was born into the Anglo-Irish landowning gentry, his status as a Roman Catholic made him something of an outsider, particularly in his dealings with Protestant British colonial elites, whether in Barbados, Hong Kong, or Mauritius.

Indeed, his earliest contributions as a Member of Parliament in 1860 pertained to the temporal power of the Pope, and unfolding events in Italy.

Speaking at length in the House of Commons on 26 July 1860 about British civil and military forces in India, Hennessy urged a shift in policies so that "the military administration of India would be conducted with greater skill, with more economy, and, as a natural result of a higher educational standard, with a greater regard for the feelings and interests of the Native population.

Indeed, recent events furnished us with the most conclusive evidence that many of the British officers, entrusted with grave authority in India, had, from an ignorance of popular customs and a disregard of national habits and traditions, given great cause of complaint and encouragement to disaffection.

As long as we send out officers to India who seem inclined to treat the Natives as slaves, who seem unable or unwilling to appreciate the noble qualities, of that unfortunate people, and who add the grossest military outrages and insults to the civil misgovernment and financial burdens we have imposed upon them, so long will our rule in India be a blot upon civilization".

During his tenure, Hennessy realised that the Chinese people, who were treated as second-class citizens up to that time, had developed an increasingly important influence on the Hong Kong economy.

With that in mind, he lifted the ban that forbade Chinese people from buying lands, constructing buildings, and operate businesses in the Central District.

He appointed the first Chinese member (Ng Choy, who would later become the Minister of Foreign Affairs of the Republic of China) to the Legislative Council.