The shake-up also sees the departure of the managing director, John Parry.
Mr Mason, a permanent fixture in the booms and busts of the British post-war property scene, will be replaced by Geoffrey Maitland Smith, chairman of Sears, who has been a non-executive director for three years. Ronald Spinney, who resigned as deputy chairman and joint managing director of Greycoat in February 1992, becomes chief executive.
The changes come just four months after Mr Mason, 72, said he intended to stay in his post until at least the end of 1994 to allow him to bed down a successor for Mr Parry, who was due to retire when he reached 60 next March.
Yesterday, however, Mr Parry said that the search for his replacement had been under way for a year.
Hammerson has been the subject of persistent bid speculation in recent months. There have also been suggestions that Standard Life, which saved it from a pounds 1.3bn bid by Rodamco four years ago and still holds a 23.6 per cent stake, is dissatisfied and would be prepared to sell to a bidder.
Hammerson denied that the management changes were due to pressure from investors, while David Simpson, a director of Standard Life, said: 'Obviously the change is a matter for the board.'
But he added that the knowledge that Mr Parry was retiring meant there was 'a hole in the management, which was worrying. We are glad it has now been filled.'
The changes were welcomed in the City, where there has been growing concern about Hammerson's ability to recover from the property slump. Although there is widespread admiration for Mr Mason - who created the company after joining up with Lew Hammerson in 1949 - his age and Mr Parry's imminent retirement meant there was a growing desire for a change of management.
Mr Spinney, one of the co-founders of Greycoat, is widely respected in the City despite the fact that his former company has been severely affected by the recession.
His departure was believed to be both to save management overheads and because his skills are in development, which Greycoat could no longer afford.
Mr Maitland Smith, who will take up his post at the annual meeting in June, said Mr Spinney's first task would be to conduct a three-month review of the company covering the geographical spread, the mix of property types and the local management. This will form the base for a strategic plan for the group's future.
He pointed out that the average size of investment is far larger at Hammerson than its rivals, with about 80 properties accounting for the pounds 1.6m portfolio. 'That gives ease of management, but there is a difficulty in being flexible,' he said.
The effect of the recession on Hammerson was demonstrated in the results for 1992, also released yesterday, which showed pre-tax profits down 44 per cent at pounds 55.5m and earnings per share halved to 11p. The final dividend was cut from 17p to 6.5p making a total of 10p (20.5p).
Net asset value also slumped 30 per cent to 445p as a property revaluation wiped pounds 380m off its portfolio. Worst affected was Canada, which accounts for a quarter of its assets.
Borrowings rose pounds 21m to pounds 845m despite pounds 188m of disposals, partly due to sterling's devaluation. That, coupled with the fall in asset value, pushed gearing up from 78 to 114 per cent. The shares closed 23p down at 331p.