Grenfell continued to work underground until 1916, when he was appointed the miners agent for the Western Region of the South Wales Area. He became active in the local Labour Party in 1916; and in 1920 he was adopted prospective candidate for the
Gower constituency. He was elected
Member of Parliament (MP) in
a by-election in 1922, and held the seat until 1959. He acted as chairman of the
Welsh Parliamentary Labour Party, and in
Winston Churchill's coalition government during World War II he served as
Secretary for Mines at the
Board of Trade (1940–45), in which capacity he argued for the nationalisation of the
coal industry, and reiterated the call in his 1947 book,
Coal. From 1948 to 1951 Grenfell was Chairman of the Welsh Tourist Holiday Board; and while holding that post he saw
Gower become the first area of Britain to be designated an
Area of Outstanding Natural Beauty. In 1951 Grenfell was sworn of the
Privy Council. In 1953 Grenfell became the first Labour politician to hold the title "
Father of the House", as the longest continual serving Parliamentarian. Churchill had entering Parliament before him but his time as an MP was not continuous. In 1954, at Churchill's 80th birthday ceremony in
Westminster Hall, Grenfell (as Father of the House) presented Churchill with an illuminated book signed by nearly every member of Parliament. Grenfell said he had "a personal regard and deep respect for a great colleague", and added: "None of us will ever forget his great example to Parliament and to the nation. He has been a great leader in our greatest national trial." Grenfell held office in many local bodies, and was made an Honorary Freeman of Swansea for his contribution to public service. A bust of him now stands in the
Swansea Guildhall. He was succeeded as Member of Parliament for Gower by his former agent, County Councillor
Ifor Davies of Gowerton, who held the Gower seat until his death in 1982. His brother, William John Grenfell, was for a number of years a member of
Llwchwr Urban District Council, representing the Pontybrenin ward. ==Death==