In a statement Johnston said he was delighted to join the firm. He said: "MDA has made huge strides in the last few years and I pay tribute to Simon Metcalf and his colleagues for their contribution."
A vote on the future of Headlam, who joined £12.5m turnover MDA last October, will be held at a reconvened MDA annual general meeting at 11am in Fairfield Halls, south London. STG and Fairfax nominated their four alternative directors at the firm's annual meeting last month.
MDA shareholder Stefan Allesch-Taylor backed the move to appoint the directors, saying it would give MDA industry experience.
Allesch-Taylor said: "The combination of Charles and David gives the company leadership that has hands-on experience."
MDA doesn’t need a bunch of city slickers coming in
He claimed that MDA did not need much corporate experience. He said: "It doesn't need a bunch of city slickers coming in. It needs people who can meet client needs."
Allesch-Taylor said the management changes came after talks between shareholders and MDA management.
He said: "A tripartite agreement was made between us, the senior management and the MDA board that the company needed this to go forward."
He insisted the changes in the composition of the board would not lead to a change in ownership. He said: "MDA is not for sale."