The move from DBD Media follows a period of major changes to the online offering by Mr & Mrs Smith including the launch of flight search functionality with meta search provider Kayak, a loyalty scheme and user reviews.
Greenlight recently won the account for fellow luxury travel brand Abercrombie & Kent.
Warren Cowan, chief executive at Greenlight, said: “The luxury market is constantly expanding and our role to make the site more accessible will aid this growth.”
Mr & Mrs Smith marketing director Tamara Heber-Percy added: “Greenlight has a great track record when it comes to the travel sector.
“Their specialist in house technologies are really impressive and are ideal for taking our paid search activity to the next level.”