The painting, which was unveiled Tuesday, will hang outside the Blue Room, where mayors hold press conferences and sign bills.
The portrait joins a collection 44 other New York City mayors in the nearly 200-year-old City Hall.
Giuliani's portrait is the 45th mayor in the collection.
It was painted by Everett Raymond Kinstler and features the former mayor in a pinstripe suit and red tie, with his trademark toothy grin and dimples.
Giuliani was painted during several sittings - a few times in 2006, before his failed bid for the Republican presidential nomination, and again this year.