Mr McNabb was the chairman and chief executive of mutual fund management giant Vanguard Group.
Before being appointed as CEO in 2008, and chairman in 2010, Mr McNabb held various leadership roles at Vanguard.
He served as CEO for nine years before retiring in 2017, and continued as chairman until 2018. During his tenure as CEO of the group, Vanguard's assets under management grew close to five-fold to $5.3 trillion. He also oversaw the expansion of the group's advice service.
Mr McNabb has also held a number of other non-executive roles. He is a board member of New York Stock Exchange listed groups IBM and UnitedHealth Group, as well as global legal services firm Axiom.
Tilney Smith & Williamson was created by the merger of Tilney Group and Smith & Williamson which completed in September last year. Since then the group has been on an acquisition drive with new hires including Financial Planners.
The combined group is now responsible for over £50bn in client assets under management.
Will Samuel, chairman of Tilney Smith & Williamson, said: “Bill McNabb has an outstanding track record as a global heavyweight in the investment management industry and he is a well-known champion of good governance.
"We are delighted that Bill is joining the Board of Tilney Smith & Williamson following the recent merger. The combination has created a unique business with an unrivalled proposition in the UK market, able to support clients with advice on both their personal wealth management needs and their business interests. Bill’s experience and insight will prove invaluable to Tilney Smith & Williamson as we complete the integration and embark on the next stage of growth and development.”