Green, previously head of corporate, was seen as the obvious candidate to replace Andrew Soundy and beat off the challenge of a number of other candidates thought to include former banking head John May.
Soundy was expected to stay on until next April, but is now leaving on 1 December, when Green will take over.
One of Green's priorities will be to stem the tide of defections from the firm. It emerged last week that energy partner Paul Griffin is joining US firm Cadwalader Wickershaw & Taft to head its UK project finance and privatisation practice.
Green said: “It is a disquieting time for us. My job is to build on the very strong core business we have and to build on our quality people.”
He confirmed he had been “supportive” of the Clifford Chance merger, but described speculation that his firm was looking for another merger within 18 months as “extraordinary”. He added: “During the election the question of a merger didn't come into it.”
Griffin – the latest defector – was responsible for bringing British Gas to the firm from Denton Hall two years ago. He will leave eight partners at the energy department.
Griffin declined to comment on his move but it is believed Cadwaladers offered him an attractive financial package and greater freedom to develop his project finance work than Ashursts.
Cadwalader plans to build an eight to 10-lawyer team around Griffin, who is thought to be bringing several assistants from Ashursts with him. Clifford Chance corporate finance assistant Vicky Hazeldon has also just joined Cadwaladers – bringing the total number of lawyers in the all-English law office to 24.