England's largest postal sorting office, based in central London, has had £32m worth of machinery installed.
It is hoped the new equipment, which sorts letters at a rate of more than 30mph, at the Mount Pleasant depot will help improve delivery times in London.
Recent figures show Royal Mail missed its delivery performance targets in 12 areas in the city.
Royal Mail said the redevelopment had put it in a good position to "deliver a quality service".
Some two million letters and parcels are processed at Mount Pleasant every day.
The four new machines can sort up to 45,000 letters per hour, compared with the 25,000 letters which the previous 17 machines could each manage.
The company's London regional operations director, Ian Songhurt, said: "It's put us in a really good situation going forward.
"We have invested in the best machinery out there; it's very accurate, so we're in a great position to deliver great quality service for our customers."
Robert Hammond, from Postal Policy Regulation at Consumer Focus, said the redevelopment was a welcome move which was "long overdue".
"Royal Mail needed to modernise; it needed to bring its practices into the 21st Century, and so it's a really very welcomed move as far as we are concerned," he said.
He said he was hopeful that consumers would see an improvement as London "sometimes struggled" to meet its delivery targets.
The next stage of the site's redevelopment will see the exterior restored and land sold off for for new homes.