Monsell served as the Sheriff of County Limerick in 1835. In 1847, he was elected Member of Parliament for County Limerick as a Liberal, and represented the constituency until 1874. In 1850, he became a Roman Catholic and thereafter took a prominent part in Catholic affairs, especially in Parliament. As a friend of Wiseman, Newman, Montalambert, W. G. Ward, and other eminent Catholics, he was intimately acquainted with the various interests of the Church, and his parliamentary position was often of great advantage to the Church.