During his 10 years as chairman and CEO of ACE Limited, Duperreault transformed it into global multi-line insurance and reinsurance powerhouse.

In his five years as president and CEO of Marsh & McLennan Companies, Duperreault returned it to profitability and growth, rebuilt morale and reputation, and revived the confidence of investors and clients.

In 2017, AIG, still underperforming after its long saga following the 2008 financial crisis, called upon Duperreault to turn around the company following then-CEO Peter Hancock's March 2017 resignation.

[4] A Catholic single mother, after his birth she moved with her 5-month-old son to Trenton, New Jersey where her sister lived, and worked as an accountant.

[10] Duperreault had originally thought of becoming a professor of mathematics but, newly married, he decided he wanted a better income than an academic life could provide.

[12][10] AIG at the time was a midsize property-casualty insurer, and was in the early stages of its transformation by CEO Maurice R. "Hank" Greenberg into one of the world's largest financial-services firms.

[13][20][4][21] In October 1994 Duperreault became the chairman, president, and CEO of ACE Limited, which at the time was a boutique specialty insurance company based in Bermuda with 50 employees.

[38] Duperreault's leadership of ACE for 10 years has been credited as instrumental in transforming Bermuda from an island nation with a small, reinsurance-focused market into a global insurance center.

[26][42][43][44] Due to that 2004-2005 insurance probe by New York Attorney General Eliot Spitzer, which alleged MMC had falsified and rigged bids on contracts and favored insurers at the expense of clients in return for higher commissions, the company had also sustained reputational and morale damage, lost business, and talent loss.

[45][22][5][27][42][43] Additional problems included excess capacity, lawsuits, organization issues, and very low brokerage margins,[41][27][44] and shareholders were insisting that the company be broken up.

[26][46] He also quickly built a strong new executive management team at the corporate and operating levels, implemented greater expense discipline across the company, and evaluated the overall business for fit and synergy.

[5] While continuing to reorganize and streamline MMC,[41][27] Duperreault also acquired and integrated a number of smaller companies to grow the business.

[13][27] He also established Marsh & McLennan Agency, aimed at serving the insurance and advisory needs of smaller and emerging U.S. companies,[22][44][52] and grew it substantially via acquisitions.

[47] He had streamlined costs, the company, the workforce, and service delivery,[44][27] returned MMC to profitability and growth,[27][43][56][57][47] rebuilt morale and reputation,[22][5][27] and revived the confidence of investors and clients.

[61][5][4][60] The focus of Hamilton is applying advanced data analytics and cutting-edge data-science techniques to the property-casualty insurance industry, in order to determine which risks to take and at what price.

[78][79] In May 2017, Duperreault was hired as CEO of AIG,[13] following Peter Hancock's March 2017 announcement to step down under pressure after what had been a significant period of disappointing financial results.

[80][81] Since late 2008 the AIG conglomerate had sold off many of its businesses to pay off the $185 billion U.S. government bailout it received during the 2008 financial crisis.

[12][94] Four months later Duperreault acquired Bermuda-based Validus Holdings Ltd.[83] The acquisition added business areas AIG didn't have, including reinsurance, a Lloyd's of London syndicate, insurance-linked securities, a specialist in U.S. small commercial excess and surplus underwriting, and crop insurance.

[102][83] He let his business heads set their own budgets and performance goals, gave underwriters more autonomy, and also took steps to make the job environment more amenable to younger, tech-savvy employees.

[108] In June 2019 the company formed AIG Re, which consolidated its reinsurance operations, including Validus Re, AlphaCat, and Talbot Treaty, into one global business.

[111][112][113] Also in August 2019, Duperreault and Zaffino announced "AIG 200", a three-year company-wide program to improve its operational processes and infrastructure and substantially lower its expenses, by shedding legacy processes and excess manual interventions, modernizing and digitizing workflows, and unifying the company's operations.

[143] In 2004 he established the Duperreault Fellowship, an annual scholarship which supports the professional development of Bermuda residents working or studying in the field of substance abuse.

[143][144] In 2013 he chaired a non-paid six-month task force in Bermuda called the Spending and Government Efficiency (SAGE) Commission.

"[143] Duperreault claims the impact of his faith was most apparent when he was offered the opportunity to rehabilitate Marsh & McLennan Companies, which had been suffering organizational and financial woes.