Ofgem said it imposed the fine after the company repeatedly failed to connect potential customers, in new property developments and large housing estates, quickly enough.
Sarah Harrison, Ofgem's managing director for corporate affairs, said: ‘Customers should not have to accept poor service in any part of the energy market.'
EDF's licence states that it must present new customers with a connection offer within 90 days, but an Ofgem investigation found this requirement had not been met.
EDF said it "greatly regrets the delays experienced by some customers".
The company supplies electricity to around 8m customers across London, East Anglia and south-east England.
Last month, Ofgem warned the six biggest gas and electricity providers to improve their customer service levels. It followed research that showed less than 25% of customers were happy with the way their complaints were handled.