Arnold, who has worked at the top-10 QS for eight years, will join G&T in November as a partner in the consultancy division in London.
Arnold will be replaced at F+A by Andrew Williams, a former partner at the firm for 15 years. Williams is returning to F+A after a brief stint at architect HOK Sport.
Although Kevin’s departure is indeed sad, we are fortunate to have had his commitment
F+A chairman Guy Leonard
In a statement on Tuesday, F+A chairman Guy Leonard said: "Although Kevin's departure is indeed sad news, we have been fortunate to have had his commitment and contribution to our business. We are delighted at Andrew's return, confident that he will be a great leader for F+A."
Wyatt said: "We tried to do this as honourably as we could. We have great respect for Motts as a company and would not wish to damage it. We are very excited Kevin is joining us."
Market sources claimed that Arnold was looking to return to a partnership after the merger by F+A with Mott MacDonald, which created a 7000-strong, £400m-turnover multidisciplinary business.