The move came as Wimpey decided to restructure its UK operation, merging Wimpey Homes and McLean Homes, which is expected to saved £15m a year.
Job losses are expected to take place in management and administrative positions, particularly in the land-buying and product development departments. This is expected to lead to the firm closing a number of its 29 regional offices. McLean managing director Keith Cushen will take over as managing director of the combined businesses.
Johnson told Building that the move was necessary to improve the group's performance after Wimpey's 1996 acquisition of McLean Homes, formerly the housebuilding arm of Tarmac.
He said: "When I joined Wimpey in October, the message from shareholders was that the move was never completed. No costs had been saved and an extra group of functions made the business more complicated. If we can, we need to find a way of rationalising. I am unlocking the box of change."
Johnson said staff were being kept informed of changes. He said: "Some areas containing both McLean and Wimpey operations had up to four regional offices. They were not achieving critical mass. We can manage this confidently and take into account their needs."
Under the move, regional offices that survive the cull will be given more responsibility. The Wimpey head office in Hammersmith, west London, will be closed and moved to a location between London and Birmingham.
Both trade names will be kept. He said: "Building good homes is what matters. We won't build fewer homes. People building them will have access to greater experience across the group. We will not be losing site managers or deputy site managers."
Brownfield developments are now thought to have an increasingly high profile within the firm. Johnson said he hoped the restructuring would help the firm move faster towards contributing to the government target of 60% of all developments on brownfield land by 2016.
Johnson, formerly chairman of Rugby Group, joined Wimpey after the sudden resignation of Dennis Brant in October. Sector analysts welcomed his arrival, saying he had a reputation for making radical structural changes.