The board seat for Trian is another example of the surging power of activist fund managers.
Trian might have still gotten a board seat even without BNYM's consent; activist investors have scored at least a partial victory, either by a vote or by a settlement, in 72 percent of all shareholder votes this year, according to FactSet SharkWatch data noted by The Wall Street Journal. The number was 63 percent in 2013.
Trian isn't the only activist hedge fund making a bet on BNYM. Trian owns 2.6 percent of the bank, but Marcato Capital Management also disclosed a 1.6 percent stake last month. Marcato is the 12th-largest shareholder, with 17.7 million shares as of Sept. 30; Trian is seventh with 28.8 million shares. Viking Global Investors, another hedge fund with a more passive style, also owns 16.7 million shares.
Read MoreActivist hedge fund Marcato takes stake in BNY Mellon
"We have had valuable discussions with Ed and Trian over the past several months about our progress towards improving our financial performance and capitalizing on the expanding opportunities in the markets we serve," Gerald Hassell, BNY Mellon's chairman and CEO, said in a statement Tuesday. "Ed brings to our board the perspective of a large and engaged shareholder in BNY Mellon and has a track record of working collaboratively with other boards and management teams."
Trian was founded in 2005 by Garden, Nelson Peltz and Peter May and manages $10 billion today.
Trian has experience working with financial service companies given past investments in State Street, Lazard and now Legg Mason. Peltz recently resigned from the Legg Mason board but Trian is still a large shareholder.
Read MoreNelson Peltz resigns from Legg Mason board