The company's roots are in the 1824 foundation of the Belgian life insurer Assurances Générales (now AG Insurance). In 1990 AG took over the Netherlands-based firm AMEV/VSB to form Fortis. AMEV/VSB had itself been formed earlier that year by the combination of a savings bank, VSB (Verenigde Spaarbank), and an insurer, AMEV, which followed the end of legislation preventing mergers between banks and insurers. AMEV had originally been founded in Utrecht in 1920 as Algemeene Maatschappij tot Exploitatie van Verzekeringsmaatschappijen (English: General Society for Operation of Insurance).
After its creation, Fortis extended its activities to private and investment banking and to asset management, establishing subsidiaries around the world, and by 2007 it had become the 20th largest business in the world by revenue. That year Fortis agreed to purchase ABN AMRO jointly with Banco Santander and Royal Bank of Scotland Group, but the 2007-10 financial crisis exacerbated problems with financing its part of the acquisition and prompted fears of insolvency. Considered "too big to fail", Fortis received an €11.2 billion bailout from the Benelux governments and saw its retail banking operations in Belgium sold to BNP Paribas, and its insurance and banking subsidiaries in the Netherlands nationalised.
The remaining assets of the company, consisting principally of insurance operations but also including some distressed assets, were rebranded Fortis Holding. In April 2010 its shareholders agreed a formal change of name to Ageas SA/NV, with ownership of the Fortis brand passing to BNP Paribas.
On July 13, 2018, the Amsterdam Court of Appeal (Gerechtshof Amsterdam) approved a €1.3 billion collective settlement of claims asserted on behalf of shareholders of the former Fortis (now Ageas), under the authority of the Dutch Act on Collective Settlement of Mass Claims (Wet Collectieve Afwikkeling van Massaschade) or WCAM.