John J. "Jack" Brennan (born 1954) has served as Chairman of The Vanguard Group from 1996 to 2009, and was Vanguard CEO from 1996 until being succeeded by William McNabb in 2008.
Brennan joined Vanguard in 1982. For many years he served as the company's president alongside John C. Bogle who was Vanguard's founder and CEO. When Bogle stepped down from his chairmanship in 1996, Brennan was his hand-picked successor. There is no debate about the total lack of cordiality of Bogle's departure and the nature of Brennan's role in excluding Bogle from Vanguard's board. Since then Brennan has remained committed to Vanguard's long time strategy of providing low cost no-load mutual funds to its investors.
He was also a previous chairman of the Investment Company Institute, a trade organization representing mutual fund companies.
Brennan was a member of Sigma Nu fraternity.
Brennan's father, Frank Brennan, ran Union Warren Savings Bank.