During the latter period he was involved with the foundation of the Communist Party of Great Britain (CPGB), participating in a Provisional Committee which prepared the Founding Conference, in London on 31 July – 1 August 1920.
In 1922 Deer was elected as Labour member of Lincoln City Council where he served for sixteen years and was Mayor in 1933–34.
While serving as Sheriff of Lincoln in 1943–44, Deer was made a member of the Order of British Empire in the New Years' Honours list of 1944.
Deer became popular among fellow MPs, and often took up transport issues which he knew about from his trade union work.
At the 1950 general election, Deer changed constituencies to Newark, which was adjacent to Lincoln and slightly better for Labour.