The Lord Mayor's Show is a spectacular event which includes a river pageant, procession in a gold coach, and fireworks.
Where is the best place to watch the Lord Mayors show procession from
It's a fairly long route, stretching all the way from Mansion House, down past St. Paul's, and on to the Royal Courts of Justice, so you can see the procession all along that route.
But the actual action (the ceremonial parts) take place outside St. Paul's and the Royal Courts of Justice. If I had to recommend one of those two then I would go for St. Paul's, because there's a big semi-circular area in front of the steps. You will likely be able to see more of what's going on -- but the grandstand seats cost money. You need to book them -- purchase.tickets.com/buy/TicketPurchase ... id=8267298. [Note: The grandstands aren't outside the front steps, they are down the side of the cathedral along St. Paul's Churchyard.]
The main ceremony takes place outside the Royal Courts of Justice.
But remember there's also some boats to see along the river, plus the fireworks. The best place for the boats is wherever you can see Tower Bridge, because it will open up to let them through. The fireworks are best seen along the riverbank between Waterloo Bridge and Blackfriars.