Mansion House is the official residence of the Lord Mayor of London. It is a Grade I listed building.
 
It is used for some of the City of London's official functions, including two annual white tie dinners. At the Easter banquet, the main speaker is the Foreign Secretary, who then receives a reply from the Dean of the Diplomatic Corps, i.e. the longest-serving ambassador. In early June, it is the turn of the Chancellor of the Exchequer to give his "Mansion House Speech" about the state of the British economy.
 
Interesting facts...
The Mansion House was paid for in an unusual way: the City authorities, all Church of England men, found a way to tax those of other Christian denominations, particularly the Rational Dissenters.