Williamson beat Andrew Rayner, Thomson's marketing director, to take the job; it is unclear whether Rayner will remain at the firm.
TUI Travel is based at First Choice's offices in Crawley and managed by First Choice chief executive Peter Long. The combined business encompasses more than 170 brands, and is expected to generate sales of £12bn.
Williamson has held several roles at First Choice, including product director.
First Choice declined to comment.