The five-year contract covers about 6,500 train-card advertising panels, with audiences of up to 80 million passengers a year.
The win increases KBH's share of First Group's London train network in its advertising portfolio.
The company will be upgrading its panels on First Capital Connect trains to provide two standard sizes in square and landscape formats, to be rolled out on all its ad routes.
New routes now covered in the contract see trains enter London from towns including Brighton, Bedford, Peterborough and Cambridge, as well as to and from Luton and Gatwick airports.
As well as advertising on the routes, KBH will provide FCC with identical frames to provide information such as maps and safety notices onboard the trains, creating a standard look to the carriages.