The contract had initially been due to finish at the end of September 2013 and FirstGroup was given a 28-week extension last year.
It has now been confirmed that the First Capital Connect franchise will operate until September 13, 2014.
FirstGroup is on the shortlist for the new Thameslink, Southern and Great Northern franchise, which will replace Capital Connect.
FirstGroup chief executive Tim O'Toole said: "This agreement allows us to continue our improvements over the next six months, focusing in particular on customer information provision and assistance."
The government's franchise tendering programme is getting back on track after the 2012 debacle when FirstGroup had the West Coast Main Line contract awarded and then rescinded due to flaws in the Department of Transport process.
Battling a stretched balance sheet, FirstGroup then conducted a £615 million fundraising last year.
In October FirstGroup received a two-year extension on its operation of the Great Western franchise.
FirstGroup has been shortlisted for the ScotRail contract, a line it currently operates, and the new 15-year Caledonian Sleeper franchise.
The company is also in the running for the East Coast Mainline franchise where its rivals include a Stagecoach-Virgin Group joint venture.