McKell became Minister of Justice after Labor won the 1920 election and retained the position until the Government was defeated in 1922. When Labor regained office under John Thomas (Jack) Lang's Leadership in 1925, McKell returned to the Justice portfolio. He also became Lang's assistant at Treasury. In May 1927, Lang reconstructed his Cabinet with loyalists. McKell at first retained his post but was dropped on 8 June. When Lang won the 1930 election, McKell became a Minister but was relegated to the minor portfolio of Local Government. In June 1931, McKell again became Justice Minister, where he remained until Lang's dismissal by Governor Game in May 1932.