Byers, who is global head of restructuring for Grant Thornton International, has led a range of regulatory assignments for the European Commission, International Monetary Fund and European Central Bank.
Romanovitch is also part of the leadership board with the people & culture portfolio, a role which she will retain.
Geoff Davies will continue his role as head of advisory in the regions.
Scott Barnes, CEO, Grant Thornton UK, said, "The advisory practice has delivered a stellar performance over the last few years, reporting a 21.7% rise in turnover in the last financial year. Mark and Sacha have some excellent plans to build on this success and develop a clear offer that we know clients will appreciate.”
The mid-tier firm expanded its advisory pactice last year with the acquisition of the UK financial services advisory arm of consultancy Navigant .The 45-strong Navigant team included three new partners and nine directors.