Wilson was former RHS curator at Hyde Hall and Harlow Carr and more recently has been working two days a week as head of creative development for the RHS while recording his new TV series Landscape Man, due out on Channel 4 in spring 2010. He was on a fixed term two-year contract at the RHS.
The Landscape Agency designed Wilson's pet project at Harlow Carr, its learning centre.
Wilson said: "After 10 rewarding years with the RHS I'm thrilled to be joining The Landscape Agency. I was hugely impressed, as a client, with the way in which the practice went about developing the RHS garden masterplans, and I can't wait to contribute to the many exciting projects they are developing."
The Landscape Agency's current clients include the RHS, National Trust, English Heritage, Yorkshire Sculpture Park and Lowther Castle in Cumbria.
Wilson's appointment will also see him heading The Landscape Agency's new London office in Clerkenwell, working alongside existing offices in York and Warwick.
Subscribe to Horticulture Week for more news, more in-depth features and more technical and market info.