The London-based insurance broker will receive an initial US$4.3bn from Blackstone, with up to US$500mln becoming subsequently due, depending on how the business performs.
The acquired business is the largest benefits administration platform in the US, servicing around one in six of the US's working population.
AON confirmed the sale on the same day it revealed better than expected fourth quarter results.
Earnings per share (EPS) eased 11% to US$1.87 from a year earlier, but with exceptional items stripped out rose 13% to US2.56. Analysts had pencilled in a figure of US$2.49 for adjusted EPS.