The company - which controls 46 per cent of Newspaper Publishing, owner of the Independent and Independent on Sunday - said yesterday that Mr Montgomery carried out the transaction last Friday, two days after the announcement of interim pre-tax profits, before exceptional costs, of pounds 39m.
Mr Montgomery exercised options over 400,000 shares granted at 61p on 23 October 1992 and sold them at 213p a share. He also sold 150,000 existing shares at the same price.
The company added that Mr Montgomery retains options over 579,409 shares at 61p and options over 163,755 at 229p, and also has a holding of 50,999 shares in the company.
In March of this year, Mr Montgomery made a profit of pounds 780,000 when he exercised and sold options on 500,000 shares the day after Mirror Group reported 1995 pre-tax profits of pounds 87.2m.
At last Wednesday's interim results announcement, Mr Montgomery said that the company was "an efficiently run media business capable of further expansion".
Shareholders were told that they would receive a 12.5 per cent rise in the interim dividend to 1.35p.