The 27-year-old's long-term deal commits him to Irish for the peak of his career, suggesting he is unlikely to seek another move away from the Aviva Premiership outfit.
Coventry-born Geraghty made 71 appearances at Irish between 2004 and 2009 after graduating from the club's academy.
Strained relations came with his move to Northampton Saints in 2009, and Geraghty then progressed to a stint in the French Top 14 with Brive.
The six-cap England playmaker returned to Irish in 2012 though, and has since restored his status as a mainstay under rugby director Brian Smith.
Geraghty was joined by prop Halani Aulika, scrum-half Darren Allinson and England Under-18s wing Tom Fowlie in signing new deals with Irish.
This four-strong group to recommit follows George Skivington, Ian Humphreys, David Paice, Jon Fisher and Topsy Ojo in extending their Irish contracts.
Irish boss Smith said securing another clutch of first-team players on new contracts underlines the club's growing stability after a board takeover last month.
"It's a great way to start the New Year and extremely positive news for the club and our supporters," he told London Irish's official website.
"I'm delighted that all four of them have committed their futures.
"It has been an exciting period for the club off the field and the players are excited by where we want to go on the field.
"Tom Fowlie is going to be a huge part of this club going forward and it is great to see Academy talent coming through the club and going on to represent the first team."