Manning first attempted to enter NSW politics when he stood unsuccessfully as the Liberal Reform Party candidate for the New South Wales Legislative Assemblyseat of Phillip at the 1910 election, the seat of King at the 1913 election, and again as the United Australia Party candidate in King at the 1932 election. Despite this, Manning was nominated, and subsequently appointed for a life term, by Premier Bertram Stevens to the Legislative Council of New South Wales and then joined Stevens' second ministry as Attorney General and, by virtue of his now leadership of the government members in the Legislative Council, Vice-President of the Executive Council, on 17 June 1932. Manning, who had been interested in reforming of the council towards being a non-partisan House of review, involved himself in drafting a proposal, with support from Stevens, for an Upper House of sixty members, elected to a statewide electorate by a joint session of Parliament. His proposal, informed by the significant criticism of the elitist nature of the current Council, especially from the Labor Party, was subsequently adopted by parliament and approved at a statewide referendum in 1933. Consequently, Manning, whose life appointment was now void, was elected to the new council for a six-year term on 7 November 1933, and retained his cabinet positions until government's defeat at the 1941 election. On 2 January 1939 he was knighted as a Knight Commander of the Order of the British Empire "In recognition of service to politics and government" From 1934 to 1948 he was a fellow of the Senate of the University of Sydney. In opposition, Manning was unofficially regarded as the leader of the non-Labor members but, affirmed of his beliefs regarding a non-partisan council, refused the role when offered to him. He further opposed in 1950 a proposal that the Liberal Party should form a party in the council, though his ideas for a non-partisan council came under attack from the Liberals and a Labor Party reanimating its longstanding policy to abolish the upper house. Elected to a second six-year term on 14 March 1946, he retired from the council in April 1958 and was granted the title "The Honourable" for life.
Later life
Manning was chairman of MLC Limited from 1945 to 1961. By the end of his life considered "one of Australia's leading constitutional law experts", and survived by his two daughters, Manning died at his home following an Asthma attack in Randwick on 3 May 1963 and was buried in the Catholic section of Northern Suburbs cemetery after a service at St Mary's Cathedral, Sydney.