The win partially fills the gap caused by the loss of the long-standing MEC film distribution client UIP, which moved to ZenithOptimedia late last year.
MEC was awarded the account without a formal pitch and will take on planning and buying with immediate effect. The brief was previously held by the specialist entertainment agency Target NMI.
Rob Norman, the chairman of MEC, said: "Redbus is a highly entrepreneurial, commercial and creative business and we are delighted to have this chance to prove we can work for a strong independent as well as a studio such as UIP. This appointment adds to our relationship with Redbus, which, through its ventures arm, is a shareholder in Video Island, another client of ours."
Simon Franks, the Redbus chief executive, said: "MEC committed a lot of energy and creativity to its proposal and we have decided to see if it can put this into action. We have decided to see if MEC's scale and UIP experience could create a competitive advantage for us."