He was returned unopposed in a by-election caused by the declaration of his predecessor Charles Leach as insane. Leach remains the only MP to be deprived of his seat having been declared of unsound mind. A supporter of the Lloyd George coalition, Mallalieu was re-elected in 1918 without Tory opposition. When the Tories stood in 1922 following the end of the coalition Mallalieu finished third, losing to Labour’s Philip Snowdon. Mallalieu’s son, Lance, also represented Colne Valley as a Liberal from 1931 to 1935.