In 1820, he settled in London, where he became the secretary of the Committee of West Indian Merchants in which role he advocated strongly on behalf of slave-owning sugar plantation owners.
[1] It was a lucrative post that made him a wealthy man and left him with plenty of time to indulge his passion for chess.
[5] At that time, the world's strongest player was the French aristocrat Louis-Charles Mahé de La Bourdonnais.
Between June and October 1834, La Bourdonnais and McDonnell played a series of six matches, a total of eighty-five games, at the Westminster Chess Club in London.
In the summer of 1835, his condition worsened and he died in London on 15 September 1835 before his match with La Bourdonnais could be resumed.