Since 1980, he has been married to high school girlfriend Terri McGuinty, who is an elementary school teacher. The couple have one daughter and three sons.

His father, Dalton Sr., served as Member of Provincial Parliament (MPP) for Ottawa South until his death in 1990. Dalton Jr. won the Liberal Party’s nomination for Ottawa South for the provincial election of 1990 and was elected to the Legislative Assembly of Ontario as the MPP for his father’s former riding.

The Liberal government of David Peterson was defeated by the social democratic Ontario New Democratic Party (NDP) in that election. In opposition, McGuinty served as the Liberal Party’s critic for Energy, Environment and Colleges and Universities. He was re-elected in Ottawa South in the 1995 provincial election without much difficulty. The Liberals maintained their status as the official opposition amid a provincial swing from the NDP to the Progressive Conservatives.

McGuinty’s supporters in his 1996 leadership bid included John Manley, Murray Elston, and Bob Chiarelli. He was elected leader at the party’s convention December in a surprise victory over front-runner Gerard Kennedy.

Harris resigned in 2001 and his successor, Ernie Eves, received a short boost in the polls from his attempts to move the PC Party to the centre.

The 2003 North America blackout gave Eves increased exposure and rallied some support for his party. He called an election immediately after the blackout, and polling showed that the previous Liberal lead had narrowed to a tie in the first week. The rise in Tory support was short-lived. The Liberals took a commanding lead in the campaign’s second week, and remained in that position until election day.

McGuinty took office as Premier and Minister of Intergovernmental Affairs on October 23, 2003.

The new government called the Legislature back in session in late 2003. The government brought in auto insurance reforms (including a price cap), rolled-back a series of corporate and personal tax cuts that had been scheduled for 2004, passed legislation that enshrined publicly funded healthcare into provincial law, hired more meat and water inspectors, opened up the provincially owned electricity companies to Freedom of Information laws and enacted a ban on partisan government advertising.

To pay for this plan, the Liberals imposed a controversial new Health Premium of $300 to $900, staggered according to income. This violated a key Liberal campaign pledge not to raise taxes, and gave the government an early reputation for breaking promises. The Liberals defended the premium by arguing that the previous government had a hidden deficit, and McGuinty claimed he needed to break his campaign pledge on taxation to fulfill his promises on other fronts. His own finance critic of the time, Gerry Phillips, had predicted that the Tories’ projected balanced budget would in fact result in a $5 billion deficit in a meeting of the Standing Committee of Estimates of the Legislature on June 3, 2003. Liberal MPP Monte Kwinter also predicted a $5 billion deficit

On May 18, 2004, Provincial Finance Minister Greg Sorbara released the McGuinty government’s first budget, the first year of a four-year plan focused on tackling four deficits the Liberals claim the previous Tories left behind: the “health deficit”, the “education deficit”, the “infrastructure deficit” and the “fiscal deficit”.

In late 2004, John Tory was chosen to replace Ernie Eves as leader of the Progressive Conservative Party of Ontario. A principal secretary to former PC Premier Bill Davis, Tory was regarded as more moderate than Mike Harris and the mostly rural MPPs who made up the majority of his caucus. McGuinty’s Liberals ran a candidate against Tory during the latter’s successful bid to enter the legislature. Howard Hampton continued to lead the NDP. Though Tory out-polled McGuinty in the category of preferred premier, the Liberals held a lead over the Progressive Conservatives, while the NDP held around 20% support.

During early 2005, McGuinty called the Legislature back for a rare winter session to debate and pass several high-profile bills. The government legislated a “greenbelt” around Toronto. The size of Prince Edward Island, the Greenbelt protects a broad swath of land from development and preserves forests and farmland. In response to court decisions, the McGuinty Liberals updated legislation to reflect the change in the definition of marriage to include homosexual couples.

In late April 2005, McGuinty announced the closure of the Lakeview coal-fired generating station, one of Ontario’s largest polluters. Although the McGuinty government had promised to close all coal-burning plants by 2007, Energy Minister Dwight Duncan announced on June 14, 2005, that this was no longer possible, and that the Nanticoke Coal Plant will not close until 2009.

On May 11, 2005, the McGuinty Liberals delivered their second budget, built around the “Reaching Higher” plan for education. The second year of the four-year plan, this budget was designed to tackle to so-called “education deficit”. Investing $6.2 billion over the next four years, the budget included the largest investment in higher education in forty years. It also increased accessibility for low-income students, expanded medical school spaces, and invested in new faculty, graduate scholarships and research. The budget also broke a promise to balance the budget in 2007–08. The government instead aimed at balance in 2008–09.

On June 22, 2005, Education Minister Gerard Kennedy announced that 90–95% of Ontario students between junior kindergarten and Grade Three would be in classes of twenty students or fewer by 2007. He also acknowledged that extenuating circumstances may require slightly larger classes in some cases. Opposition critic Frank Klees accused the McGuinty government of breaking its promise to cap classroom sizes. Kennedy responded that some flexibility is always necessary, and that any reasonable person would regard a 90–95% success rate as a promise kept.

Also in June 2005, two cabinet ministers in McGuinty’s government were scrutinized for alleged improprieties. Joseph Cordiano faced calls for his resignation after revelations that he billed $17,000 for personal expenses to his riding association. These expenses included meals in Paris and Milan, and theatre tickets in London. Cordiano insisted that these expenses were related to riding activities, and refused to resign. McGuinty defended Cordiano in public, claiming he had “complete confidence” in the minister.

At around the same time, Minister of Transportation Harinder Takhar was accused of a conflict-of-interest, after visiting a company that he owned in a blind trust. Takhar acknowledged that he made “an error in judgement”, but denied any wrongdoing. Both Cordiano and Takhar were retained in their portfolios following a cabinet shuffle on June 29, 2005. The matter was sent to the provincial ethics commissioner, who on January 4, 2006, ruled that Takhar had violated Ontario’s integrity guidelines by not maintaining an arms length relationship with the trustee appointed to run his blind trust. McGuinty has defended his minister, and has rejected calls to remove him from cabinet, even after the Integrity Commissioner issued his finding.

On October 11, 2005, police raided the Sorbara Group offices — owned by Greg Sorbara and his brothers — as part of the ongoing Royal Group Technologies investigation. The warrant stated that there were reasonable grounds to believe Sorbara and other directors of Royal Group defrauded the company and shareholders when they bought land in Brampton, that was owned by a subsidiary of the Sorbara Group. Sorbara initially resisted opposition calls for him to step down, then resigned as Minister of Finance the same day. He consistently denied any knowledge of the specific allegations against him, and sued the RCMP to either clarify their case against him or withdraw their investigation. Following Sorbara’s resignation, Dwight Duncan was appointed as Minister of Finance and Chair of the Management Board. Donna Cansfield took over Duncan’s responsibility as Minister of Energy and Jim Bradley as Government House Leader.

On November 18, 2005, it was announced that Ontario’s Drive Clean emissions program was to be expanded rather than scrapped.

The 2006 budget was the third year of the four-year plan, and focused on the “infrastructure deficit”. The centrepiece was MoveOntario, a $1.2 billion investment in transportation infrastructure. $400 million was invested to build and repair roads and bridges in municipalities across Ontario.

On May 18, 2006, a judge agreed with Greg Sorbara’s contention that the Royal Canadian Mounted Police (RCPP) had erred in including his name in the search warrant. In striking Sorbara’s name from the warrant, Justice Ian Nordheimer of the Ontario Superior Court said there were inadequate grounds for police to include him in the first place. The judge was particularly scathing in his review of the RCMP probe of Sorbara. On May 23, 2006, Sorbara was reinstated as Minister of Finance, while Duncan returned to the Energy portfolio.

On August 17, 2006, Foreign Direct Investment magazine (a British magazine owned by the Financial Times) named Dalton McGuinty “personality of the year” for encouraging investment in the auto sector, for developing a plan to increase energy production, and for promoting research and innovation.

On June 14, 2006, Energy Minister Dwight Duncan announced the McGuinty government’s twenty-year electricity plan, which committed to spending $46 billion on rebuilding all of the province’s ageing nuclear reactors. The plan also made the McGuinty government the first Ontario government since the 1970s to commit to building new nuclear stations, and further postponed the schedule for closing Ontario’s coal stations to 2014. In response, Greenpeace activists occupied Energy Minister Duncan’s offices. The day after the announcement of its long-term electricity plan The Globe and Mail published a front-page story that the government had quietly passed a regulation to ‘exempt’ its energy plan from an environmental assessment.

In October 2006, the McGuinty Liberals held their last Annual General Meeting before the 2007 election. The event set in place several key elements of their reelection strategy. First, American political consultant James Carville advised Liberal activists to stick to a simple message in the next election. Second, the party elected long-time activist Gord Pheneuf as the new president. Finally, Premier McGuinty laid out the theme of the next campaign: standing up for Ontario families.

On July 26, 2007, McGuinty dismissed Mike Colle as Minister of Citizenship and Immigration following a report by the auditor general that Colle had mishandled government funds. He was criticized for giving out $32 million in government grants to immigrant and cultural groups without official applications or formal statements of purpose. In one case, the Ontario Cricket Association received $1 million when it asked for $150,000.

On October 10, 2007, McGuinty and his Ontario Liberal Party won a consecutive majority government in the 39th general provincial election. The last Liberal Party Premier to achieve such success was Mitchell Hepburn during the 1930s.

The 2009 Ontario Budget contained significant tax policy changes: McGuinty’s Minister of Finance Dwight Duncan announced plans to harmonize Ontario’s retail sales tax with the federal Goods and Services Tax, and reduce corporate and personal taxes.

On March 31, 2009, McGuinty admitted to considering of the removal of the minimum wage increase at 2010 from $9.25, to $10.25 as a mistake after the reactions that he received from the opposition and anti-poverty activists.

On October 6, 2011, the Liberals were reelected for a third term in government, though they lost their majority in the Legislature. Only winning 53 of the 107 legislative seats, the Liberals were one seat short of a majority.

McGuinty commissioned former Toronto-Dominion Bank chief economist Don Drummond to examine the province’s finances. In February 2012, Drummond released the report that stated that the province would face a $30-billion deficit by 2017-18, which was nearly double what the McGuinty government had projected. Drummond recommended severe austerity measures to curb the growth of the province’s debt, which was $215 billion at the time, from reaching $411 billion in five years. While McGuinty inherited a moderate deficit of $5 billion, it had tripled due to his progressive spending programs as well as the recent economic recession.

In 2012, the Liberals were plagued by yet another scandal. During the Ornge Air Ambulance scandal, the CEO was paid 14.1 million dollars, and the executives purchased a commercial building, and them leased it back to themselves at a higher than market rate, through a shell company.

The Liberals had hoped to regain their majority through by-elections and on September 6, 2012, two by-elections were held, one in Vaughan which the Liberal Party won, and another in Kitchener—Waterloo. Former Deputy Premier Elizabeth Witmer, a Progressive Conservative, had represented Kitchener—Waterloo for the previous four terms. Her resignation allowed the riding to elect Catherine Fife of the New Democratic Party (NDP), leaving McGuinty’s Liberals with a minority government. The NDP victory in Kitchener-Waterloo, the seat the Liberals needed to gain, was attributed in part to the backlash against Bill 115; the Liberals placed third in the riding.

After it became known that there were an additional 20,000 documents Bentley was cited by a rare contempt motion by a legislative committee and was facing a contempt motion of the Legislature when McGuinty unexpectedly announced on October 15, 2012, that he was ending the legislative session by proroguing the legislature and that he would resign as premier as soon as the Liberal Party chose his successor, in January 2013.

While there had been speculation that McGuinty would become a candidate for the federal Liberal leadership election, on October 23, 2012, he announced that he would not be doing so. McGuinty resigned from his premiership on February 11, 2013. He resigned his seat in the legislature on June 12, 2013, at the end of the legislative session, after representing Ottawa South for 23 years.

Liberal House Leader John Milloy later stated that prorogation was necessary because an impasse was reached with labour leaders and the opposition over plans to freeze all public sector wages. The opposition charged that it was done to dodge negative publicity over the investigation and criminal probe into the Ornge Air affair, the controversial decision to halt construction of two gas-fired power plants during the previous election, and the subsequent threats by the opposition to vote on finding Cabinet ministers in Contempt of Parliament for withholding from the Legislature information related to halting the projects. In early 2013, Onley explained in an interview conducted by the Toronto Star that, though he and McGuinty discussed the matter, among others, before he granted the prorogation, he could only follow the constitution and adhere to the principles of responsible government; only if the premier were “trying to subvert democracy” could Onley have refused the advice and, as Onley put it, “something that’s politically controversial doesn’t fit that category. Doesn’t even come close…. It’s up to the politicians to work out the political process, the political decision-making that is behind prorogation—and the fallout after prorogation.” On the subject of the lack of a date on which the Legislature would be summoned to return, the Lieutenant-Governor said he had no guide; the Legislature’s standing orders outline that a specific date must be set, but the Legislative Assembly Act does not, and precedents are inconsistent.

In June 2013, McGuinty was named a senior fellow for the fall semester at Harvard University’s Weatherhead Center for International Affairs.

McGuinty published a memoir with Dundurn Press in 2014 called Dalton McGuinty: Making a Difference about his life in politics.