 He was originally elected outside the parties in electoral coalition with the conservative party Højre, but joined the Free Conservatives in February 1904.Along with the rest of the Free Conservatives, Sonne joined the Conservative People's Party in 1915, but he left the party again in June 1917 as a protest against the party's intention of pulling Minister without Portfolio Christian Rotbøll out of the Cabinet of Zahle II.Sonne was Minister for Agriculture for a short term after the Easter Crisis of 1920 as the only member of the Cabinet of Friis who was nationally known beforehand.