Mervyn Dunnington-Jefferson was born on 10 February 1850. He was the son of Reverend Joseph Dunnington-Jefferson and Anna Mervinia Vavasour. He married Louisa Dorothy Barry, daughter of Reverend John Barry, in 1881. He died on 21 March 1912 at age 62.
He was educated at Eton College, Windsor, Berkshire, England. He gained the rank of Captain in the 33rd Regiment. He held the office of Justice of the Peace (J.P.)