After working closely with the Landscape Agency on the new Harlow Carr Learning Centre through his part-time role as RHS head of creative development, Wilson was offered a permanent job.
He is to become associate director of design at the Landscape Agency from November and will set up a new London office in Cowcross Street, in the City. In addition, he plans to continue in a voluntary capacity at the RHS - where his contract was due to end in June.
"I'd been talking to the Landscape Agency ever since I started my new role with the RHS because it is part-time and we hoped I could work for them during my non-RHS time," explained Wilson.
"As things developed, this great opportunity opened up. If I was going to do it then now was the right time, with a new family and being keen to secure their future as much as anything else."
RHS director of gardens and estates Jill Cherry said she would miss Wilson greatly and added: "I'm delighted that he is keen to maintain his links with the RHS."
Garden presenter Wilson is planning a second series of upcoming television show Landscape Man.