Born in St. Thomas, Ontario, Hepburn attended school in Elgin County and hoped to become a lawyer. His educational career ended abruptly, however, when someone threw an apple at visiting dignitary Sir Adam Beck knocking his silk top hat off of his head. Hepburn was accused of the deed, denied it, but refused to identify the culprit. Refusing to apologise he walked out of his high school and obtained a job as a bank clerk at the Canadian Bank of Commerce where he worked from 1913 to 1917 eventually becoming an accountant at the bank's Winnipeg branch.
At the outbreak of World War I, Hepburn was already enlisted in the 34th Fort Garry Horse, but was unable to obtain his parents' consent to sign up for the Canadian Expeditionary Force. He became a lieutenant in the 25th Elgin Regiment of the Canadian Militia, and was conscripted to the 1st (Western Ontario) Battalion in 1918. He transferred to the Royal Air Force and was sent to Deseronto for training, but injuries in an automobile accident that summer, followed by influenza in the fall, kept him from active service. He returned to St. Thomas to tend the family's onion farm.
After the war, Hepburn joined the United Farmers of Ontario (UFO) helping to start its branch in Elgin County, but by the mid-1920s he switched to the Liberal Party. In the 1926 election, he was elected to the Canadian House of Commons as a representative of Elgin West, and was overwhelmingly re-elected in the 1930 election.
Later that year he became leader of the Liberal Party of Ontario. His support of farmers and free trade, and his former membership in the UFO allowed him to attract Harry Nixon's rump of UFO Members of the Legislative Assembly (MLAs) into the Liberal Party (as Liberal-Progressives). This and the Great Depression led to the defeat of the unpopular Conservative premier George Stewart Henry in the 1934 provincial election. His stance against the prohibition of alcohol allowed him to break the Liberal Party from the militant prohibitionist stance that had helped reduce it to a rural, Protestant south western Ontario rump in the 1920s.
Hepburn represented a type of agrarian democracy that detested Toryism and valued oratory. He once saw a pile of manure situated in a village square, and proceeded to jump on top of it to give a speech, apologizing to the crowd for speaking from a Tory platform. He also used the same line when standing on a manure spreader, only to have a heckler shout, "Well, wind 'er up Mitch, because she's never carried a bigger load!"
On his death, the Toronto Star observed:
Hepburn's premiership achieved international attention, which merited his appearance on Time magazine's cover in 1937.
As premier, Hepburn undertook a number of measures which enhanced his reputation as the practitioner of a highly vigorous style. In a public show of austerity, he closed Chorley Park, the residence of the Lieutenant Governor of Ontario, auctioned off the chauffeur driven limousines used by the previous Conservative cabinet, and fired many civil servants. In order to improve the Province's welfare, he gave money to mining industries in Northern Ontario, and introduced compulsory milk pasteurization (in so doing, he has been credited with virtually wiping out bovine tuberculosis in the province). Breaking with the temperance stance of previous Liberal governments, Hepburn expanded the availability of liquor by allowing hotels to sell beer and wine.
The Industrial Standards Act, which emulated the US National Industrial Recovery Act, was introduced in 1935 to set minimum wages and working conditions by industry and geographic area. It was described by then Minister of Labour David Croll as "the most controversial piece of legislation now on the Statute Books of the Province," and it came about after federal efforts instituted under R.B. Bennett's "New Deal" were declared unconstitutional.
The government also made international news by making the Dionne quintuplets wards of the provincial crown in response to public outrage of plans by promoters to exploit the infants by putting them on display at the Chicago World's Fair. The Legislative Assembly passed legislation in that regard, subsequently replaced in 1944 (which was not repealed until 2006).
As Treasurer of Ontario, Hepburn adopted a more aggressive approach in the collection of succession duty on large estates, which resulted in millions of dollars in extra government revenues. He made no apologies for doing so, as he noted in a speech in 1938:
One estate that was of particular focus in this campaign was that of the late John Rudolphus Booth, who had died in 1925. Although succession duties of $4.28 million ($60,400,000 in current terms) were paid in 1927, Hepburn subsequently claimed more in 1937 and had the Legislative Assembly of Ontario pass the necessary legislation to overcome the legal obstacles. Booth's heirs eventually paid another $3 million ($50,000,000 in current terms) in 1939.
As part of his drive to cut government spending, the Power Commission Act, 1935 was passed to cancel contracts that the Hydro-Electric Power Commission of Ontario had signed between 1926 and 1930 for delivery of electricity from power plants in Quebec, declaring them to be "void and unenforceable." This move temporarily shut Ontario out of world bond markets. The Act was declared to be ultra vires by the Ontario Court of Appeal in 1937 as being legislation in derogation of extraprovincial rights (although later jurisprudence has suggested that the Court may have overreached in its rulings). Many such contracts were later renegotiated at lower volumes and prices.
Hepburn took an aggressive position with respect to timber licenses in Northern Ontario that were being held by companies that would not (or could not) cut wood on them. In that regard, in 1936 the Forest Resources Regulation Act was passed that granted the government broad powers for mandating minimum production quotas, maximum limits in line with good forestry practice, reducing licensed acreages where they were in excess of requirements, and increasing stumpage fees on companies "operating or carrying on business in a manner detrimental to the public interest." Great Lakes Paper saw its holdings reduced from 23,085 square kilometres (5,704,000 acres) to 3,668 square kilometres (906,000 acres), and was assessed a $500,000 penalty ($8,700,000 in current terms) for refusing to participate in a minimum price agreement set up by the Ontario and Quebec governments.
In 1937, the Settlers' Pulpwood Protection Act was passed to govern the supply of pulpwood from private lands, together with fixing quotas and prices to be followed.
The Crown Timber Act's provisions — in effect since the time of Arthur Sturgis Hardy — requiring logs to be manufactured as timber before being exported from the Province were relaxed by order in council in 1936, in order to create employment opportunities in the logging industry.
When the Abitibi Power and Paper Company, in receivership since 1932, was ordered into liquidation in 1940, Hepburn appointed a Royal Commission to investigate the matter, in order to determine the best course of resolution. The Legislative Assembly imposed a moratorium on liquidation proceedings in 1941, which was ultimately upheld by the Judicial Committee of the Privy Council in 1943. The Commission's recommended plan was accepted by all creditors, and it would emerge from receivership in 1946, one of the longest such receiverships in Canadian history.
Although in later years Hepburn was to form a Liberal-Labour alliance with the Communist Party of Canada, yet as Premier he was opposed to unions and refused to let the CIO form unions in Ontario. On April 8, 1937, the CIO-backed General Motors plant in Oshawa went on strike, demanding 8-hour workdays, a seniority system, and recognition of their CIO-affiliated United Auto Workers union. The strikers were also supported by the Co-operative Commonwealth Federation, Canada's left-wing party at the time. Hepburn, at the time professing a deep concern about radicals among auto workers and supported by the owners of the plant and General Motors, organized a volunteer police force to help him put down the strike when Prime Minister William Lyon Mackenzie King refused to send the Royal Canadian Mounted Police. This force was somewhat derisively known as "Hepburn's Hussars," or the "Sons of Mitches". Cabinet ministers who disagreed with Hepburn over the issue were forced to resign. However, the strike held out, and Hepburn capitulated on April 23, 1937.
Hepburn remained a bitter opponent of Mackenzie King after the strike, and harshly criticized King's war effort in 1940 after the outbreak of World War II introducing a resolution into the Ontario legislature which was passed 44 to 10, that accused the federal government of mishandling the war effort. The Conservative opposition voted unanimously for the resolution but the motion split the governing Liberals with nine members of Hepburn's caucus voting against and others leaving the chamber before the vote. Hepburn thought Canada should be doing more to support the war, and helped organize the military districts in Ontario, encouraging men to volunteer when Mackenzie King chose not to introduce conscription.
Hepburn supported Mackenzie King's opponent, Arthur Meighen, in a by-election in Toronto in 1942, notwithstanding Hepburn's later alliance with the Communist Party of Canada. Meighen's unusual source of support did not bring him to success, as he lost the by-election because the Liberals did not run a candidate and the prime minister ordered party resources to be sent to the CCF candidate. However, King was politically much stronger than Hepburn and federal Liberal supporters, as well as those who thought an erratically driven rift between the provincial and federal parties was suicidal, called for him to step down. Hepburn ultimately resigned as Premier in October 1942, but continued to serve as Treasurer of Ontario and party leader until the following year.
Although Gordon Daniel Conant had become Premier of the province, many people continued to think that was in name only. Senior cabinet ministers such as Provincial Secretary Harry Nixon resigned, demanding a leadership convention. Due to pressure from both provincial and federal Liberals, one was held in May 1943. Hepburn finally tendered his resignation as leader (by telegram), and Nixon was elected the new party leader, and was thus appointed as Premier.
The Liberals under Nixon were routed soon after in the 1943 Ontario election, falling to third party status behind the Progressive Conservatives led by George Drew, and the Co-operative Commonwealth Federation led by Ted Jolliffe. Hepburn himself was re-elected in his riding as an Independent Liberal while calling for a Liberal-Conservative coalition against the burgeoning CCF. The Liberal caucus unanimously asked Hepburn to resume the party's leadership in 1944.
Now branding Drew's Conservatives as the greatest menace to Canada, he reversed his earlier criticism of Mackenzie King's war effort and campaigned for Liberal candidate General Andrew McNaughton in a 1945 federal by-election. Provincially, his earlier, vehement doubts about radicals among auto workers now muted, Hepburn formed a Liberal-Labour alliance with the Communist Party of Canada (at the time known as the Labor-Progressive Party) for the 1945 Ontario election, but lost his own seat in the Legislature.
Hepburn retired to his farm in St. Thomas, where he died of a heart attack in 1953. His funeral was attended by five former premiers, and Rev. Harry Scott Rodney observed in his eulogy:
Hepburn was the first Liberal to become Premier since George William Ross, and was the last Liberal Premier to win two successive majority terms until Dalton McGuinty.
In 2008 he had a school named after him only miles away from his family's farm. It was officially opened in January 2009.