In the winter of 1678–9, Arnold was restored to the bench at the request of Worcester's son, Lord Herbert. Arnold, Lord Herbert, William Morgan, and the Bishop of Llandaff, began hunting down Roman Catholics in Monmouthshire, with Arnold offering a personal bounty of £200 to anybody who would capture a Catholic priest and send them to him. Amongst those brought to him was the Jesuit David Lewis, and the college of Jesuits at the Cwm was also attacked. In the second general election of 1679, Arnold stood for Monmouth but lost to Lord Herbert and was admitted to the Green Ribbon Club in November of that year. The election result was overturned on petition in 1680, and Arnold was seated for Monmouth instead of Lord Herbert. He continued to fight the Catholics and complained in parliament about the Monmouthshire justices failing to enforce the Penal Laws. Arnold fell into disrepute with the Catholic Herberts of Coldbrook around this time. Arnold was also responsible for prosecuting and executing Philip Evans on the testimony of three witnesses he found.
