The shake-up leaves the Carillion board with just three executive members and continues its move away from its traditional contracting roots to longer-term contracts, following its demerger from Tarmac two years ago.
But McDonough will follow this policy without PFI and services director McEwan, who has overseen the PFI division's growth to a £470m-turnover business over the past five years.
McEwan left the group immediately after the news was announced last Friday and construction director Sharples is to retire after 30 years with the company.
Operations director Roger Robinson, finance director Chris Girling and McDonough remain on the streamlined board, which came into effect on Tuesday. The divisional managing directors will report directly to McDonough.
McEwan said he was surprised by McDonough's decision but accepted it and insisted that the split was amicable.
He said: "It was a shock and I would have loved to have carried on the development of PFI and services but he wanted to do it himself. He wants to go for a flatter structure and I can fully understand that." McEwan added that he wanted to stay in the PFI sector but was unsure where he would go.
McDonough, who joined the firm at the start of the year from US facilities management group Johnson Controls, said Carillion would continue its move towards generating more income from its PFI and services-related activities "alongside our well-established construction businesses".
Construction still accounts for about 80% of Carillion's turnover, which stood at £1.9bn last year.
The City this week welcomed the board shake-up, saying the new structure made sense.
Peel Hunt analyst Stephen Rawlinson said it was obvious that McDonough wanted a flatter and leaner management set-up.
He said: "Streamlining the board will make it more flexible." Carillion's share price rose 0.5p to 186.5p soon after the announcement last Friday, which was 2% lower than at the start of last week. By early this week, the price was 187.5p.