In January 1910, he was elected MP for Brighton, being re-elected in December. His majority of 4,000 was, he was to comment later, unusually high in a period when average electorates were not large. In 1911, the Hon. Walter Rice became 7th Baron Dynevor on the death of his father. Thereafter, he became increasingly involved with the politics of his native Carmarthenshire. Vice-president of the Carmarthenshire Conservatives in 1912, he was also President of the West Wales Conservatives to 1914, when Conservative re-organisation saw him become President of the South Wales Conservatives, a post he held until 1938.
During the Great War, Lord Dynevor served in the Ministry of Munitions from 1916. Thereafter, he served on the UnionistDevolution Committee, considering the recommendations of the Speaker's Conference on Devolution.
In his locality, he served as a member of Llandeilo Rural District Council representing Llandyfeisant, a very small parish which sat largely within the Dynevor Castle estate. In 1919, he was elected to the Carmarthenshire County Council for Llandeilo, capturing a traditionally Liberal seat as an independent. He retained this seat until increasing deafness forced him to resign in 1935.
Throughout the inter-war years, Lord Dynevor was a key figure in Welsh Conservative politics, as well as the Carmarthenshire territorials. In 1928, Lord Dynevor became Lord Lieutenant of Carmarthenshire. Forced to resign from his offices in 1938 due to increasing deafness, Lord Dynevor was praised for his record of public service.
Writings
In addition to his public duties, Lord Dynevor wrote two books, My Reminiscences, published in 1937, which comprises a short memoir, and a book on the trees in the park at Dynevor.
Legacy
Walter Street, in Ammanford, Carmarthenshire is named for the 7th Baron. Margaret Street was named for his wife.
In 1916 displaying Welshness had become sufficiently fashionable for Lord Dynevor to adopt (by royal licence) the older, Welsh form of his surname, "Rhys". In 1906 Rice Street in Betws had been named after him.
Dexter a griffin per fess Or and Argent wings addorsed and inverted tail between the legs, sinister a talbot Argent collared flory counterflory Gules ears Ermine and charged on the shoulder with a trefoil slipped Vert.