"I've been fortunate to have had an enjoyable and successful career as a player at club and international level," said Catt, who will remain in a coaching role with the club.
"London Irish has given me the opportunity to make the transition from playing to coaching and the time is right for me to focus full time on that role.
"I hope to help the club maintain the progress it has made in recent seasons and compete successfully in the Guinness Premiership and Heineken Cup."
Catt toured twice with the Lions in 1997 and 2001and, although he only played seven times, the World Cup winning utility back experienced almost every emotion with the best of the four Home Unions.
After missing out on selection for the original touring party in 1997, Catt won a late call up and was an unused replacement in the second Test win over South Africa before starting the third and final international in his homeland on that memorable tour.
He toured again four years later and was tipped for a starting spot against the Wallabies but was forced home early with a back injury that had threatened his original participation.
Catt returned to action for England in November of 2001 but then spent exactly two years in the rugby wilderness. Not wanted by a successful England, Catt's international career appeared to be coming to as disappointing an end as his second Lions adventure.
Again, Catt fought back. Just as he had silenced the jeering brigade earlier in his England days, he performed so well at domestic level that Sir Clive Woodward ended his international exile by naming him in his 2003 World Cup squad.
Catt made the most of his unexpected opportunity, featuring in the pool matches against South Africa, Samoa and Uruguay, before turning the quarter-final against Wales on its head with a commanding display as a second-half replacement in Brisbane.
Whereas he left Australia disheartened and empty handed with the Lions in 2001, Catt flew home in 2003 as World Cup winner and a far more contented individual.
A second international exile between 2004 and 2006 saw Catt miss out on selection for the 2005 Lions tour but he returned to England colours in time to be an important member of yet another incredible World Cup adventure.
Catt's fourth World Cup in a row brought another final appearance and, although England finished runners up to South Africa in Paris, Catt's international career finished on the biggest stage of all, rather than petering out without so much as a flicker as it had so often threatened to do.
Domestically, Catt starred for Bath and London Irish, winning the Heineken Cup with the former in 1998 and making close to a century of appearances with the later since following his move to the Exiles six years ago.
He was the club's Player of the Season in 2006 and received the RPA Players' Player of the Season award in the same year. Last year he was awarded with the O2 Special Merit Award at the RPA Rugby Players awards.