McMahon was born in
Ireland. He moved to
South America in 1818 to serve in
Simón Bolívar's "
British Legions". In 1819 he left and migrated to
British Jamaica where he worked as a bookkeeper (
foreman) and
overseer. He lived there for eighteen years and worked on twenty-four different plantations. He was unemployed for a period after criticising the treatment of slaves, and served in suppressing the
Baptist War, a
slave rebellion of 1831–32. McMahon later became an avowed
abolitionist, and wrote an account of his experiences, entitled
Jamaica Plantership. ==References==