In December last year, Cairns resigned from the then HHCL United and signed a contract with Beattie McGuinness Bungay to run a number of the agency's accounts. These included the Sky One business BMB won from Wieden & Kennedy last November. However, HHCL made a counter offer and Cairns returned to the agency.
The changes occurred in the wake of the exit of Nick Howarth, United's former chief executive, who quit to join Clemmow Hornby Inge.
In her new role, Cairns will be briefed to develop creative partnerships with clients in non-traditional media, within both the WPP group and the United network.
Jim Kelly, the agency's new joint managing partner, said: "Rhona's talents were clearly evident to the London market; it's time that they were recognised here at United London also."
Sid McGrath, United's managing director, added: "Rhona has been outstanding on our Sky business and fully deserves this promotion. As United London explores more non-traditional media opportunities for its clients, Rhona will create alliances with like-minded agencies."
Cairns joined the agency in 2000. She began her career at BMP DDB, where she worked on Doritos, Johnson & Johnson and Thomson Holidays. She then moved to M&C Saatchi to work on Shell, Gallaher, Save the Children and Qantas.