M.J. Coldwell was born in Seaton, England, on December 2, 1888. Educated in England, Coldwell arrived in Canada in 1910 to take up his first teaching appointment in Alberta. The next year he was hired as a principal in Sedley. Later he was a school principal in North Regina and Regina until 1934. Active in several teachers' federations from the mid-1920s until the mid-1930s, he served as president of the Saskatchewan Teachers' Alliance and president of the Canadian Teachers' Federation. Coldwell became an alderman in 1921 and served four terms until 1932.
In 1925 Coldwell was a federal candidate for the Progressive Party and also ran in 1934 as a provincial candidate for the Saskatchewan Farmer-Labour Party. A year later, Coldwell, campaigning for the federal CCF in Rosetown-Biggar, was victorious and would go on to win four subsequent elections in 1940, 1945, 1953, and 1957, until losing in the Tory landslide of 1958.
In 1931 Coldwell was president of the Independent Labour Party and was selected the first leader of the Saskatchewan Farmer-Labour Party in 1932, which was renamed the Co-operative Commonwealth Federation in 1934. George Williams replaced him as provincial leader after the 1934 election.
Following Woodsworth's death in 1942, Coldwell was elected the national CCF's leader and remained in the post until 1960. Under Coldwell's tenure, the CCF achieved its greatest gains federally in seats and vote in 1945. Later, the aging and ill Coldwell was no match for the prairie populism of John Diefenbaker and the party's vote and seats fell dramatically in 1958. Following this setback, social democracy would be reborn as the NDP.
Coldwell was respected by his peers in other political parties and was offered Cabinet posts in provincial and federal governments, Speakership of the House of Commons and a Senate seat. He served as a member of the Canadian delegation at the creation of the United Nations. Coldwell was named to the Privy Council (1964) and made a Companion of the Order of Canada (1967). The Douglas-Coldwell Foundation, a left-wing educational organization, is named after two remarkable Saskatchewan political figures.
Coldwell's distinguished record of public service was achieved while caring for an invalid wife for two decades. Coldwell died in Ottawa on August 25, 1974.