Thompson, who was previously Portland's managing director, assumes responsibility for the UK operation from the chairman Dennis Sullivan, who will now concentrate on overseas operations.
Thompson has worked at Portland since 1995 - his first job as the general manager of Portland Polska saw him move to Warsaw to set up the office.
He returned to London in November 1999 as the managing director of the UK arm after a stint as the international business director.
Sullivan will lead a phase of overseas expansion in Europe. Portland Outdoor has offices in the UK, Ireland, Hungary, India and Malaysia.
Thompson said his promotion was both a recognition of the growth in the size of the UK agency and a sign that Portland wanted to grow its non-WPP group business.
"While size is important, we are widening the gap over our competitors with our continued investment in proprietary tools and the quality of our people," Thompson said.
"Our business is now centred very clearly on WPP business, non-affiliated agencies and direct clients, with the right people on board to service their needs. We are focusing on adding value to their advertising communication strategy."