In May Withers boosted its London office with funds partner John Greenword from offshore firm Appleby.
McDermott specialises in media advice to wealthy individuals as well as companies. Her clients include the Barclay brothers and Tube operator Tube Lines.
McDermott joined Addleshaws in 2004 after spending 23 years at Lovells.
Addleshaws managing partner Mark Jones said: “We wish her well. It’s an entirely amicable move. She joined us to help with the public law practice. It’s just one of those things that didn’t work out.”
He said the firm would not close the practice. “In terms of advising people on reputation management, it’s something we’ve done in the past and will continue to do in the future,” he said. “The fact that Jennifer has left is no indication by us that this is an area that we’re moving out of.”
Jones added that litigation partner David Engel has been instructed by Alexander Litvinenko murder suspect Andrei Lugovoi for defamation and reputation work.