Leigh will leave the firm later this summer, having handled the top finance job since 2011.
A statement from an FTI spokesman read: "I can confirm that Tom will be leaving FTI over the summer after eight years with the firm. During that time he has led several acquisitions, helped integrate Strategic Communications within FTI, and built a finance infrastructure to support our 650+ SC staff members globally as well as the 500+ FTI employees in London.
"We thank him for all of his contributions to FTI as we wish him well for his next chapter."
Leigh, who previously worked at NHS supplier Patientline, joined FTI Consulting as group controller and in 2007 became finance director of its UK business. He was promoted to the group-level job four years later.
In May, FTI Consulting’s strategic comms unit reported an 8.9 per cent organic revenue decrease to £25.6m for Q1.
This compared with overall revenue growth for the company, which includes four other advisory arms including technology and litigation consulting, of 4.5 per cent to £252.4m.
At the time Steven Gunby, who took over as president and CEO of FTI Consulting in January, stated that he expected the firm would begin "meeting the aspirations" it has in a relatively short time.
FTI declined to comment on whether Leigh, who is thought to be heading to an undisclosed new job, would be replaced.