Off-duty firefighters sprang into action when they spotted a fire at a barber's shop on Bedale Street in Southwark.
They forced entry into the shop and put the blaze out using an extinguisher inside.
One fire engine from Dowgate Fire Station also attended along with fire investigators, who believe the fire was caused by a hairdryer which had been left on.
A London Fire Brigade spokesperson said: “It’s possible the hairdryer had overheated and cut out and was then put back without being switched off. It may have restarted once it had cooled down.
“This incident is a timely reminder to make sure you always switch your hairdryers and straighteners off, even if they have cut out.
“They should also be left to cool on a heat proof surface.”
The Brigade was called at 2106 and the incident was over for the attending crew at 2120.