Mansion House is the official residence of the Lord Mayor of London. It is used for some of the City of London's official functions, including an annual dinner, hosted by the Lord Mayor, at which the Chancellor of the Exchequer customarily gives a speech – his "Mansion House Speech" – about the state of the British economy. The Guildhall is another venue used for important City functions. It is a Grade I Listed Building.
More information on the Wikipedia page [1], and the Corporation of London website is [2].