O’Leary had aspired to become a photographer, but on the advice of his stepfather attended university, where he continued to develop his interest in business and investing. He received an honours bachelor’s degree in environmental studies and psychology from the University of Waterloo in 1977 and an MBA in entrepreneurship from the Ivey Business School at the University of Western Ontario in 1980.

In 1979, between the first and second years of his MBA program, O’Leary was selected for an internship at Nabisco in Downtown Toronto, and then worked as an assistant brand manager for Nabisco’s cat food brand. O’Leary credits his later success at The Learning Company to the skills he developed in marketing during his days at Nabisco.

After selling his share of SET, O’Leary started Softkey in a Toronto basement in 1986, along with business partners John Freeman and Gary Babcock. The company was a publisher and distributor of CD-ROM-based personal computer software for Windows and Macintosh computers. A major financial backer who had committed $250,000 in development capital to the company backed out the day before signing the documents and delivering his cheque, which left O’Leary looking for funding to support the fledgling business. He used the proceeds from the sale of his share of SET and convinced his mother to lend him $10,000 in seed capital to establish SoftKey Software Products.

Softkey weathered stiff competition from other software companies in the late 1980s and prospered throughout the 1990s. By 1993, Softkey had become a major consolidator in the educational software market, acquiring rivals such as WordStar and Spinnaker Software. In 1995, Softkey acquired The Learning Company (TLC) for $606 million, adopting its name, and moved its headquarters to Cambridge, Massachusetts.

TLC lost $105 million (US) in 1998 on revenues $800 million and suffered losses over the previous two years. TLC bought its former rival Brøderbund in June 1998 for $416 million.

In 1999, TLC was acquired by Mattel for US$4.2 billion. Sales and earnings for Mattel soon dropped, and O’Leary departed Mattel. The purchase by Mattel was later called one of the most disastrous acquisitions in recent history. Following the acquisition, Mattel experienced a US$105 million loss where management had projected a $50 million profit. Mattel’s stock dropped, wiping out US$3 billion of shareholder value in a single day. Mattel’s shareholders later filed a class-action lawsuit accusing Mattel executives, O’Leary, and former TLC CEO Michael Perik of misleading investors about the health of TLC and the benefits of its acquisition. The lawsuit alleged that TLC used accounting tricks to hide losses and inflate quarterly revenues. O’Leary and his defendants disputed all of the charges. Mattel paid $122 million to settle the lawsuit in 2003. O’Leary blamed the technology meltdown and a culture clash of management of the two companies for the failure of the acquisition.

In 2003, O’Leary became a co-investor and director at StorageNow Holdings, a Canadian developer of climate-controlled storage facilities, a company controlled by Reza Satchu and Asif Satchu. StorageNow became the operator of storage services in Canada, with facilities in 11 cities, and was acquired by in Storage REIT in March 2007 for $110 million. He sold his shares, originally worth $500,000, for more than $4.5 million.

In May 2005, Reza Satchu and O’Leary’s operating partner, Wheeler, filed a $10-million wrongful-dismissal lawsuit, charging that they had altered an agreed-upon compensation deal and illegally reduced Wheeler’s share of the profits. O’Leary and Satchu claimed Wheeler failed to reach performance targets. The case was settled out of court.

In 2006, O’Leary appeared as one of the five venture capitalists on the then-new show Dragons’ Den on CBC, the Canadian instalment of the international Dragons’ Den format. On the show, O’Leary developed a persona as a blunt, abrasive investor, who at one point told a contestant who started crying, “Money doesn’t care. Your tears don’t add any value.” This television persona was encouraged by executive producer Stuart Coxe, who during the first two seasons occasionally asked O’Leary to be “more evil”. Dragons’ Den became one of the most-watched shows in CBC history, with around two million viewers per episode. Coxe attributed the show’s success in large part to O’Leary’s presence.

In March 2007, O’Leary joined the advisory board of Genstar Capital, a private equity firm that focuses on investing in healthcare services, industrial technology, business services and software. Genstar Capital appointed O’Leary to its Strategic Advisory Board to seek new investment opportunities for its $1.2 billion fund.

In 2008, O’Leary co-founded O’Leary Funds Inc., a mutual fund company focused on global yield investing. He is the company’s chairman and lead investor, while his brother Shane O’Leary serves as the director. The fund’s assets under management grew from $400 million in 2011 to $1.2-billion in 2012. The fund’s primary manager was Stanton Asset Management, a firm controlled by the husband-and-wife team of Connor O’Brien and Louise Ann Poirier.

In 2008, O’Leary worked as a co-host for the Discovery Channel’s Discovery Project Earth, a show that explores innovative ways to reverse climate change.

In 2009, the American version of Dragons’ Den, Shark Tank, began, and Shark Tank executive producer Mark Burnett invited two of the CBC Dragons’ Den investors, O’Leary and Robert Herjavec, to appear on the show. Both have remained with Shark Tank since the beginning. For several years, they appeared on both shows, although Herjavec left Dragons’ Den in 2012, and O’Leary left in 2014. Shark Tank became a ratings hit, averaging 9 million viewers per episode at its peak in the 2014–15 season. It has also been a critical favourite, winning the Primetime Emmy Award for Outstanding Structured Reality Program three times.

In 2009, O’Leary began appearing with journalist Amanda Lang on CBC News Network’s The Lang and O’Leary Exchange. During a segment of The Lang & O’Leary Exchange on the Occupy Wall Street protests in 2011, O’Leary criticized Pulitzer Prize-winning journalist Chris Hedges for sounding “like a left-wing nutbar.” Hedges said afterwards that “it will be the last time” he would appear on the show and compared the CBC to Fox News. CBC’s ombudsman found O’Leary’s behaviour to be a violation of the public broadcaster’s journalistic standards.

In September 2011, O’Leary released his first book, Cold Hard Truth: On Business, Money & Life, in which he shares his views on entrepreneurship, business, finance, money and life. A sequel, The Cold Hard Truth on Men, Women, and Money: 50 Common Money Mistakes and How to Fix Them, was published in 2012. It focused on financial literacy and financial education as a foundation for achieving wealth. O’Leary released a followup in 2013 in which he covers subjects relating to important life choices: education, careers, marriage and family, and retirement. He discusses the obstacles of raising a family while working to provide financial security for them and gives advice for developing financial literacy in family members, saving and investing money, and managing debt and credit.

O’Leary and his wife, Linda, have been married since 1990. The couple separated in 2011, but resumed their marriage after two years. Linda now serves as the VP of Marketing for O’Leary Wines. They have two children. Trevor is a music producer and DJ while Savannah is a multimedia producer at the Huffington Post. In an interview with Inc. Magazine writer Brian D Evans O’Leary stated: “In a successful growing business, it eats your time alive. Then later in life, you can provide for your family things that many others can’t have. But because you sacrificed, you’re then given the reward of freedom.”

O’Leary’s appearances on Dragons’ Den and Shark Tank popularized the nickname “Mr. Wonderful” for him; he has said that he is often referred to by that name in public. O’Leary has said that the nickname serves both as a tongue-in-cheek reference to his reputation for being mean, as well as a reflection of his view that his blunt assessments are helpful to misguided entrepreneurs. In a 2013 interview, O’Leary implied that he could not remember how he got the nickname. He had already referred to himself as “Mr. Wonderful” in a 2006 casting video for Dragon’s Den, predating either show.

In November 2014, O’Leary Funds Management agreed to pay penalties to the Autorité des marchés financiers for violating certain technical provisions of the Securities Act. At the time of the agreement, O’Leary Funds reported that it had taken steps to correct the violations. On 15 October 2015, O’Leary Funds was sold to Canoe Financial, a private investment-management company owned by Canadian businessman W. Brett Wilson, who once was an investor with O’Leary on CBC’s Dragons’ Den.

O’Leary founded O’Leary Ventures, a private early-stage venture capital investment company, O’Leary Mortgages, O’Leary books, and O’Leary Fine Wines. In April 2014, O’Leary Mortgages closed. O’Leary’s funds have a questionable history and are said to have declined over 20% in a year, which is often a big blow to fund managers. O’Leary no longer manages outside money.

In January 2014, on The Lang and O’Leary Exchange, O’Leary remarked,

O’Leary has produced and hosted his own reality show, Redemption Inc., in which he tries to help ex-convicts start and develop their own businesses. Having also been a co-host of SqueezePlay on Bell Media’s Business News Network (BNN), he returned to the Discovery Channel on 1 September 2014 to join as a contributor for its radio and television stations such as CTV.

On 14 July 2015, O’Leary launched an ETF through O’Shares Investments, a division of his investment fund, O’Leary Funds Management LP, where O’Leary serves as chairman. A value investor, he has given advice on personal finance. He advocates portfolio diversification and suggests that investors have their age as the percentage of bonds in their portfolios (i.e., 30% in bonds and 70% in stocks for a 30-year old investor, with an increasing proportion of bonds and decreasing proportion of stocks as the investor ages). O’Leary has also “stated on many occasions that he won’t invest in a publicly traded stock unless it pays him a dividend.”

On 5 May 2015, O’Leary made an appearance on the game show Celebrity Jeopardy and received $10,000 for his charity despite finishing 3rd and in negative points after both Double Jeopardy and Final Jeopardy rounds. In September of that year, O’Leary appeared as a celebrity judge in the 95th Miss America pageant.

In January 2016, O’Leary offered to invest $1 million in the economy of Alberta in exchange for the resignation of Premier Rachel Notley and appeared with four other prospective leadership candidates at a conference for federal Conservatives in late February 2016 where he gave a presentation titled “If I Run, This is How.” During his speech he predicted that the Liberal government would fall within four years from economic collapse.

In February 2016, Maxime Bernier, a Conservative Quebecois politician, criticized O’Leary, calling him a “tourist” for wanting to be prime minister without being able to speak French. Bernier later explained that he wanted all leadership candidates to learn French and praised his fellow leadership contender Lisa Raitt, who was trying to improve her French. O’Leary stated that he was taking French lessons, and promised to learn French in time for the next federal election.

On 18 January 2017, O’Leary officially entered the Conservative leadership race. That same day, his former Dragons’ Den co-star Arlene Dickinson stated that she found O’Leary to be too “self-interested and opportunistic” to be qualified for the office of prime minister. In response, another former Dragons’ Den co-star, W. Brett Wilson, endorsed O’Leary, highlighting differences between O’Leary as a businessman and his TV persona.

On 1 February 2017, O’Leary posted a video of himself shooting in a Miami gun range. It was removed from Facebook out of respect for the funeral for three victims of the Quebec City mosque shooting on that day. It was also revealed that he was in New York promoting one of his business ventures when this occurred. O’Leary later apologized for the timing of this post.

O’Leary was a frontrunner in the polls throughout most of his run. Nevertheless, he dropped out of the leadership race on 26 April 2017, stating that, though he still thought he could win the leadership election, a lack of support for him in Quebec meant that it would be difficult for him to beat Trudeau in 2019, and that it would thus be “selfish” of him to continue. On dropping out, he endorsed Bernier, considered the other main frontrunner for the position. (Andrew Scheer eventually won the leadership election, narrowly edging out Bernier.)

In a 2017 interview with Evan Solomon, O’Leary suggested that Senators should pay money every year, instead of being paid, thus turning “a cost centre to Canada” into “a profit centre.”

In November 2018, O’Leary hired lawyer Joseph Groia and sued Elections Canada and Canada’s federal elections commissioner over campaign finance laws which limited candidates to spending only $25,000 of their own money for their leadership campaign. At the time of the lawsuit, O’Leary still owed $430,000 to creditors. O’Leary had proposed to Elections Canada that he pay off the debt now with his own money and fundraise the money later, but was rebuffed, since this would be illegal. O’Leary made public statements that the law promoted mediocrity since rich people would be discouraged from running and hurt the businesses who had pledged money for his failed leadership campaign.

On 24 August 2019, O’Leary was involved in a fatal crash on Lake Joseph in Muskoka, Ontario. A 64-year-old man and a 48-year-old woman died when a boat owned by O’Leary was involved in a collision with another boat. O’Leary said in a statement that he is cooperating with the police investigation, that he was a passenger in his boat, and that the other boat did not have its lights on and “fled the scene”. The police stated that both boats left the scene to “attend a location and both parties called 911.” On 24 September, Linda O’Leary, the operator of the O’Leary’s boat, was charged with “careless operation of a vessel” under the small vessel regulations of the Canada Shipping Act, a charge that carries a maximum 18 months imprisonment and a $1-million fine. The driver of the other boat, Richard Ruh of Orchard Park, New York, was charged with “failing to exhibit navigation light while underway.” On 11 October, the Public Prosecution Service of Canada ruled out any jail time for Linda.