MacTavish was the great-grandson of Charles Carroll of Carrollton, an American founding father, and was born into a wealthy family in Maryland.
His mother's family had married into English aristocracy, and with their support, MacTavish began a career with the British diplomatic service.
MacTavish later returned to America, where he married a daughter of General Winfield Scott, and took over a large family plantation in Maryland with around fifty slaves.
In 1834, his uncle Wellesley, then Lord-Lieutenant in Dublin, met him on a visit to Ireland and wrote that he "is receiving the education of a Catholic priest and both his understanding and his person will soon decline".
[6] He lived for some time in Paris, where he perfected his French, and then with the support of his uncle, the Marquess of Wellesley, was attached to the British diplomatic service.
[9] At the hustings, his proposer drew a link to his great-grandfather, noting that as "Charles Carroll was instrumental in effecting American independence, so he hoped that Charles Carroll MacTavish would yet be instrumental in achieving the independence of Ireland"; MacTavish said that he "long had his eye on Ireland, and though an American by birth, was an Irishman in feeling".
[10] However, an election petition was raised to challenge the result, and was adjudicated in March 1848 by a committee composed of five MPs from across the political spectrum.
[11] This point was contested over the first three days, with testimony taken from his father and a copy of an American passport provided as evidence, and a debate about whether his mother had in fact been born in England.
His youngest daughter Emily became a nun with the Order of the Visitation of Holy Mary in 1883, and her eldest sister May entered a convent in Brussels in 1887.
[19] The middle daughter, Virginia, was rumoured by gossip columnists in 1888 to be engaged to the recently widowed Henry Fitzalan-Howard, 15th Duke of Norfolk;[20] however, she never married.