Mathews was always opposed to war and militarism, and campaigned against conscription in the bitter referendums of 1916 and 1917. In 1917, he was one of several political figures prosecuted under the War Precautions Act with having "made false statements of a kind likely to affect the judgment of the electors" in the referendum, but represented himself and was acquitted. Mathews was struggling with serious health issues by the mid-1920s, and retired due to ill health at the 1931 federal election.
Mathews had a long-standing opposition to war and militarism, which he expressed through his campaigns against compulsory military service in the intense plebiscites of 1916 and 1917. He was prosecuted under the War Precautions Act in 1917 for making false statements that could sway voters' decisions but successfully defended himself and was found not guilty. Mathews's poor health led him to retire at the 1931 federal election.