The Covent Garden property's relaunch event attracted more than 1,000 guests, including key event planners, and featured Champagne, hot and cold buffets and live entertainment.
The refurbishment included restoration of the Grand Hall, which can seat more than 800 delegates theatre-style and features an Art Deco 42-foot arched ceiling, and the addition of the Tempus restaurant, coffee lounge and private dining room.
Principal Hayley Hotels and Conference Venues chairman Roger Devlin said: "Strategically it is an excellent fit within the Principal Hayley Collection and will work hand-in-hand with the nearby Hotel Russell, which is able to provide four-star accommodation and hotel amenities, as well as additional conferencing and banqueting facilities. The grand reopening has attracted many of our key clients and some of London's key event buyers and we are delighted that they could join us for this celebration."
The New Connaught Rooms was acquired by Principal Hayley in June 2009.
Principal Hayley Hotels and Conference Venues also plans to reopen the 600-room Glasgow Central Hotel in Glasgow in 2010, following a £20m transformation, as the Grand Central Hotel.