MacDonnell, Sir Richard Graves (1814–81), judge and colonial governor, was born 3 September 1814 in Dublin, the eldest son of Rev. Richard MacDonnell (qv), provost of TCD (1852–67), and his wife, Jane, second daughter of Richard Graves, dean of Ardagh. Privately tutored, he entered TCD in 1829, was a scholar (1833), and graduated BA (1835), MA (1836), LLB (1845), and LLD (1862). Called to the Irish bar in 1838 and the English bar at Lincoln's Inn in January 1841, he practised in London until appointed chief justice of Gambia on 20 July 1843. In 1847 he married Blanche Anne, third daughter of Francis Skurray, of Brighton.
In October 1847 MacDonnell was appointed governor of Gambia, where he organised several expeditions into the interior and conducted punitive campaigns against what he saw as unruly tribes impeding European trade. His next appointments were as lieutenant-governor of St Lucia in the West Indies (1852) and administrator of St Vincent (January 1853). Promoted to governor of South Australia (1855–62), he arrived at Port Adelaide in July 1855 during a time of constitutional debate: active in the responsible government question, he maintained that the colony was not yet ready for a bicameral legislature. MacDonnell advocated a single chamber legislature with appointed and elected members, where as governor he would have a powerful mediating role. As in Gambia, he took a great interest in the exploration of uncharted regions, and advocated more settlement of outback areas and the opening up of the colony's resources.
MacDonnell's lack of diplomacy in working with colonial officials, members of the administration, and the new South Australian parliament may have led to a hiatus from public office following the end of his term of governorship in 1862. He spent the next two years in England and Ireland, and in May 1864 was appointed lieutenant-governor of Nova Scotia, on the recommendation of his predecessor, the earl of Mulgrave. As in South Australia, MacDonnell arrived at a momentous time of constitutional change. He favoured a union of the maritime provinces but criticised the proposed confederation of central and maritime provinces because of the distribution of power between federal and provincial authorities. His position of lieutenant-governor became untenable when the imperial parliament supported confederation, and in October 1865 he became governor of Hong Kong. MacDonnell retired from public life because of poor health in 1872, and died 5 February 1881 at Hyères, France; he was buried in Kensal Green cemetery, London. He was made CB in 1852, knighted in 1856, and created KCMG in 1871.