The breach occurred just six months before Betfair floated on the London Stock Exchange in October 2010.
Catlett has a strong IT security pedigree, coming to Betfair in October 2009 after spending two years as Global Head of Threat and Vulnerability Management at banking giant Barclays.
Previously Catlett had been senior vice president at Bank of America, managing a team of network security experts.
Betfair insisted that Catlett had not resigned, saying he, “has chosen to return to the US to pursue an opportunity with a start-up company. We are very grateful for all the work Sean has done for the business during his time with us.
The security breach meant that between 28 March 2010 and 9 April 2010, 3.15 million account usernames with encrypted security questions were stolen, 2.9 million usernames with one or more addresses and 89,744 account usernames with bank account details were also taken
In March Betfair moved its online operations to Gibraltar, citing a more favourable tax regime.