The pair first announced plans to merge in September last year and were finally given the go-ahead by the European Competition Commission in March.
However, the Commission agreed to the deal on condition that the two companies change their existing network sharing agreements with a third smaller player, Hutchison Whampoa's 3, to "ensure that there remain sufficient competitors in the market".
Everything, Everywhere chief executive Tom Alexander revealed that both brand names will be retained, despite the merger, as both have different personalities.
Alexander said Orange offered a "premium element", while T-Mobile has "straightforward, value-orientated appeal".
"Together, we are Britain's biggest communications company, with over 30 million customers," said Alexander.
"We are Everything Everywhere – it's our name, our vision, and our ambition – and we run two of the UK's biggest brands Orange and T-Mobile. It's our vision to give our customers instant access to everything everywhere, opening up a world of endless possibilities."
The two companies will be officially become one on July 1.