Negotiations to sell the lease on the New York store, 929 Madison Avenue, are at an advanced stage.
Meanwhile, the dealership have engaged the services of Levy Real Estate of Piccadilly to dispose of Ely House, the former London residence of the bishops of Ely in Dover Street, where Mallett moved in 2012.
Levy, who recently sold the freehold of 139 New Bond Street, home to jewellery dealership SJ Phillips since 1966, will begin marketing the palatial property for a seven-figure sum this week.
Mallett chairman George Bailey, who took control of the company in difficult circumstances in September, told ATG that Mallett "no longer needs to operate from prestige four-storey premises".
New West End Location
He says the firm will continue to trade from the West End - the new location currently under review - with an emphasis on selling from premier fairs such as Masterpiece, in which Mallett retains an ownership interest.
The move to smaller premises for London's last 'antiques department store' promises a subtle change in the business model of The Fine Art Auction Group, the subsidiary of Stanley Gibbons who added Mallett to a roster of auction businesses, which include Dreweatts, Baldwin's and Bloomsbury, in an £8.6m deal in 2014.
Bailey told ATG he believes "there is great synergy between the auction and retail sides of the business", but added that "Mallett's strength is the brand and it requires a clear identity of its own".
Efforts had been under way to integrate the business with other TFAAG companies before the departure of former 'architects' Stephan Ludwig and Ian Goldbart (ATG no 2211).
Staff from Dreweatts and Bloomsbury had moved to Ely House in the hope of making the building pay its way as both an events and a saleroom location.
Last week Bailey (who is running the business alongside picture specialist James Harvey) told ATG the auction focus for fine and decorative arts will remain at Dreweatts' Donnington Priory saleroom, saying "it was "always possible to find a London venue should you wish to hold a London sale".
He said there were no plans to change the lease at Bloomsbury's headquarters in Maddox Street where regular sales would continue.
In difficult trading conditions, Mallett have struggled to turn a profit since the downturn of 2008.
A turbulent 2015 (Mallet's 150th anniversary year) included the departure of three senior executives: Henry Neville (embroiled in a case brought by US authorities against a former client) and later Giles Hutchinson Smith, CEO since 2009.
Thomas Woodham-Smith, appointed as joint co-chief executive of TFAAG, lasted just two months in the role.