The next year he founded his own agency, the Combined Registry Co.[7] As the Great Depression began, Stone reduced his workforce and improved training.

Ryan, the son of a Ford dealer in Wisconsin and a graduate of Northwestern University, had started his company as an auto credit insurer in 1964.

The firm made no US buys in 1998, but doubled its employee base with purchases including Spain's largest retail insurance broker, Gil y Carvajal, and the formation of Aon Korea.

[9] Responding to industry demands, Aon announced its new fee disclosure policy in 1999, and the company reorganised to focus on buying personal line insurance firms and to integrate its acquisitions.

That year it bought Nikols Sedgwick Group, an Italian insurance firm, and formed RiskAttack (with Zurich US), a risk analysis and financial management concern aimed at technology companies.

[10] Despite its troubles, in 2000 Aon bought Reliance Group's accident and health insurance business, as well as Actuarial Sciences Associates, a compensation and employee benefits consulting company.

[14][15] This growth strategy manifested in November 2008 when Aon announced it had acquired reinsurance intermediary and capital advisor Benfield Group Limited for $1.75 billion.

[17] Aside from drastically boosting Aon's human resources consulting capacity and entering the firm into the business process outsourcing industry, the move added 23,000 colleagues and more than $3 billion in revenue.

[21] In June 2020, Aon announced it was planning to repay the temporary 20% pay cut from 70% of employees that was published in a statement in April 2020 regarding the COVID-19 pandemic.

When the North Tower was struck by American Airlines Flight 11 at 8:46 a.m., an evacuation of Aon's offices was quickly initiated by executive Eric Eisenberg, and 924 of the estimated 1,100 Aon employees present at the time managed to get below the 77th floor before United Airlines Flight 175 crashed between Floors 77 and 85 at 9:03 a.m.[24] Many, however, did not manage to get beneath in the 17 minutes they had between the two impacts.

[27] In January 2009, Aon was fined £5.69 million in the UK by the Financial Services Authority, who stated that the fine related to the company's inadequate bribery and corruption controls, claiming that between 14 January 2005 and 30 September 2007 Aon had failed to properly assess the risks involved in its dealings with overseas firms and individuals.

[28] In December 2011, Aon Corporation paid a $16.26 million penalty to the U.S. Securities and Exchange Commission and the U.S. Department of Justice for violations of the US Foreign Corrupt Practices Act.

[29] According to the Securities and Exchange Commission, Aon's subsidiaries made improper payments of over $3.6 million to government officials and third-party facilitators in Costa Rica, Egypt, Vietnam, Indonesia, the United Arab Emirates, Myanmar and Bangladesh, between 1983 and 2007, to obtain and retain insurance contracts.

[39] On 14 November 2016, Aon acquired CoCubes an online Indian Assessment firm, facilitating hiring of entry-level engineering graduates.

[42] On 9 March 2020, Aon announced its merger with Willis Towers Watson[43][44] for nearly $30 billion in an all-stock deal that creates the world's largest insurance broker.

[47] In December 2023, Aon agreed to acquire NFP, a middle-market provider of risk, benefits, wealth and retirement plan advisory services company, for $13.4 billion.

[49] On 3 June 2009, it was reported that Aon had signed a four-year shirt sponsorship deal with English football giant Manchester United.