On his father's side, his great-great grandparents were Rev. James Haliburton (1681–1756) and Margaret Eliott, daughter of Sir William Eliott, 2nd Baronet and aunt of George Augustus Eliott, 1st Baron Heathfield. Decimus was descended from John Haliburton (1573–1627), from whom Sir Walter Scott, 1st Baronet could trace his descent on the maternal side. He was a cousin of the American judge and author Thomas Chandler Haliburton, of whom he was a close friend in London between 1838 and 1844, and thence of the lawyer and anthropologist Robert Grant Haliburton and Arthur Lawrence Haliburton, 1st Baron Haliburton.