Some ambulances were held up waiting to offload sick patients at busy Accident & Emergency units in hospitals.
At the same time, members of the public were increasingly dialling 999 for help rather than waiting to see their family doctor or travelling to A&E under their own steam.
Mr Johnson said: “The London Ambulance Service is doing an incredible job responding to Londoners at an increasingly busy time of year.
“That demand puts huge pressure on the men and women in the front line, emergency service operators, paramedics, ambulance technicians, police officers, firefighters and staff on our public transport network.
“Over the festive period and across the winter I know the public will heed the emergency services calls for restraint when it comes to calling an ambulance.”
Mr Johnson’s comments come after a memo drawn up by the Association of Ambulance Chief Executives proposed limited increases for waiting times for some serious ambulance call outs.
The proposals to deal with growing pressure on services were then leaked to a newspaper, leading to accusations that the time taken by ambulances to reach critically ill patients would double.
According to the new proposals, NHS England had agreed in principle to relax target times with a proportion of “serious but not life threatening” Red 2 incidents, which include strokes and seizures, increase from eightminutes to 19 minutes.
The only higher category is Red 1 – “immediately life-threatening” incidents such as cardiac arrest, choking and major bleeding and target for these remain unchanged.
Some doctors have warned it is that it can be very hard to tell if a situation is immediately life threatening or not over the phone when people call the emergency services.