In 1631, Jones was admitted to Lincoln's Inn in London, where he studied law; he became a member of King's Inns, Dublin in November 1640 and began practising a legal career. When the Irish Rebellion began in November 1641, he joined the Royal Army raised to suppress it, and fought under the Earl of Ormonde, who was loyal to Charles I. He took part in a number of outpost skirmishes around Lismore, County Waterford, quickly becoming known as an energetic and capable leader of cavalry. However, the outbreak of the First English Civil War in August 1642 meant Ormonde could no longer receive reinforcements or money from England, and by mid-1643, the Catholic Confederacy held most of Ireland, with the exception of Ulster, Dublin and Cork City.