Stephen Fiamma, managing partner of Jones Day's London office, says: "This is part of our strategy to build on areas of strength that the firm has worldwide as opposed to trying to build a practice in London that doesn't bear any resemblance to what we do elsewhere."
Cleveland-based Jones Day now has three corporate finance partners - Clark, Peter Faber, who joined from Simmons & Simmons last April, and Daljit Singh, who is expected to arrive from Hammond Suddards in March.
Fiamma says: "There have been many existing Jones Day clients who have made either agreed or hostile bids for companies in the UK who have come to us first and said 'If you can do this work the job is yours'.
"We have had to tell them it wasn't our expertise, but with the arrival of John, Peter and Daljit we will be able to do that work.
"We now have 43 fee earners, making us the fifth largest American firm in London, with the prospect of additional further growth as we add people to the practices of John, Peter and Daljit."
Theodore Goddard managing partner Peter Cooke says: "John goes with our best wishes and he will be joining Jones Day in due course.
"In practical terms, obviously John has a practice here but it's not going to have a huge effect.
"We have a number of other corporate finance partners who will build on what John has done here."
Theodores will merge its corporate finance and corporate departments under the leadership of head of corporate Graham Stedman.
"We have had it in mind to do that for some time. There has not been a clear distinction between them and a lot of corporate finance is done in our corporate group."