The chef's new role will include overseeing Plum + Spilt Milk, the 91-bedroom hotel's 90-cover restaurant; the GNH Bar and takeaway service KIOSK, when it officially opens in King's Cross next month.
He is currently recruiting a team to work with him and run the business on a daily basis as he splits his time between the hotel and his own restaurant Rocksalt in Folkestone.
Sargeant, who left London in 2009 after 13 years with Gordon Ramsay Holdings, said it was exciting to be returning to the capital. "It’s a fantastic opportunity to be working with Jeremy Robson and to be involved in this beautiful hotel," he added.
Ray Patterson, initially appointed head chef of Plum + Spilt Milk, will leave with Sargeant's arrival. A spokesperson for the hotel said Patterson's appointment had only ever been to help with the launch of the hotel, which opened under a soft launch on 7 April.
The newly renovated Great Northern Hotel is the work of Jeremy Robson of asset management company RAM, who has invested £40m in restoring the Grade II listed railway hotel to its former glory.
The hotel, designed by architect Lewis Cubitt in 1854, closed 12 years ago to allow for improvement work at St Pancras and King's Cross.
Its bedrooms have been designed to mimic train compartments and it offers complimentary pantries where guests can help themselves to drinks, snacks, books and magazines.