The review affected only the outdoor account - Samsung's UK media business continues to be handled by Interpublic's Initiative, and by WPP's MindShare on a global basis.
The incumbent agencies were Kinetic, which handled £1.5m worth of spend in the UK and occasional projects in Europe, while Media Keys managed more than £2m of spend in continental Europe.
The rest of the spend was handled by Cheil Communications, an advertising and media agency that works exclusively for Samsung across Europe and in which the elctronics firm owns a significant stake. Cheil managed the outdoor review on behalf of Samsung.
All of the £20m spend on outdoor, around 40% of which is believed to be in the UK, will now be planned and bought via Media Keys, with Gen implementing in the UK. Activity includes a number of long-term holdings, among them a well-known site in Piccadilly Circus.
The pair pitched against Kinetic, Posterscope and IPM. Michael Higgins, a managing partner at Gen, which is independently owned, said: "We are delighted, particularly as this is the first time we have directly pitched against established companies using the Gen model and what Media Keys can offer."
Gen works on a fees rather than a commission basis, handing the 20% commission paid by media owners for outdoor campaigns back to media agencies and clients, and using any further commission for volume of spend to buy more poster space.
Cheil's European vice-president Saul Pearce, said: "Kinetic has serviced us extremely well in the past four years. But as our business expands and needs to evolve, Media Keys has a greater synergy with us."
Kinetic declined to comment.