KBH won the contract, previously held by JCDecaux, without a pitch. It is worth about £500,000 a year in media spend and covers more than 700 train-cards, which are situated across the service's 23 trains and 92 carriages.
KBH said it would immediately begin investing in new panel frames to give onboard advertising better stand-out.
It will also offer advertisers the chance to advertise on the outside of trains for the first time. This latest win gives KBH clients access to a total of 38,000 panels.
The company's other rail concessions include C2C, First Capital Connect, First Great Western Link, Silverlink, as well as South West Trains and Wessex.
Stansted Express carries more than five million passengers every year to Stansted Airport.