After the war, Pierlot joined the Catholic Party ("Parti catholique"), the main centre-right party in Belgium and one of the three that dominated Belgian political life. The Catholic Party, which was considered the party of stability and the establishment, was extremely electorally successful during the interwar period and headed a series of coalition governments. On 23 December 1925, Pierlot entered parliament as a member of the Chamber of Deputies representing Neufchâteau-Virton but left just a week later to become a senator. He served as provincial senator for Luxembourg from 1926 to 1936 and as directly elected senator for the same province between 1936 and 1946. He received a reputation for his oratorical abilities and for personal sincerity during the late 1920s.