Spectators began to gather along the route of the race hours before the cyclists were due to speed past.
It is understood the Mayor's trip to the US was "long-planned".
Significant travel delays are expected today as the Tour de France arrives in London.
– Councillor Philippa Roe, leader of Westminster City Council
Tour could not have got off to a better start – the sight of the world’s best cyclists sprinting down The Mall is one that will linger long in the memory.
It has been an honour to host the Tour in Westminster and we wish the riders all the best as the race continues across the Channel.
Full results from Tour de France (Cambridge - London, 155 km) today are:
- Marcel Kittel (Germany / Giant) 3:38:30"
- Peter Sagan (Slovakia / Cannondale)
- Mark Renshaw (Australia / Omega Pharma - Quick-Step)
- Bryan Coquard (France / Europcar)
- Alexander Kristoff (Norway / Katusha)
- Danny van Poppel (Netherlands / Trek)
- Heinrich Haussler (Australia / IAM Cycling)
- Jose Joaquin Rojas (Spain / Movistar)
- Romain Feillu (France / Bretagne)
- Daniel Oss (Italy / BMC Racing)
Spectators at the Tour de France have been told by organisers not to run alongside riders or step into the road. A short time ago, a spectator was knocked over after getting too close to the action while trying to take a picture.