A new generation of leadership is taking over Eastern Bank Corp., the nation’s largest mutual bank. What won’t change, its leaders said, is its ownership.

The company’s long-serving chief executive is retiring, Eastern announced Tuesday, and his successor said the $9.6 billion institution will continue to remain mutually owned for the benefit of depositors and not convert to a publicly traded company, bucking a trend among smaller banks across Massachusetts.

“Eastern must maintain its mutuality,’’ said Robert Rivers, who will be promoted to Eastern’s chairman and chief executive on Jan. 1, 2017. “It is a distinct advantage for us.’’

Rivers will succeed Richard Holbrook, who has guided the bank through the recent financial crisis and helped it grow to the brink of $10 billion in assets. Holbrook said he will retire at the end of this year.

The bank also hired Quincy Miller from Citizens Financial Group, where he served as president of Massachusetts operations. Miller will start in March as vice chairman and chief banking officer and is expected to succeed Rivers as president when Rivers assumes the chairman’s role.

“Organizations continually need to reinvent themselves,’’ Holbrook said of the new leaders. “I accomplished enough to make sure the organization is on solid ground.’’

The changes come as Eastern faces growing challenges from financial technology upstarts and increased scrutiny from regulators as it expands.

Facing these pressures, several Massachusetts mutual banks in recent years have opted to sell their stock to the public to fund additional growth or make themselves more attractive for an eventual sale to a larger institution. Just last week, Randolph Bancorp announced that it was planning to covert to a stock holding company.

But being a mutually owned bank that doesn’t have to respond to stockholder demands for investment returns, Rivers said, has allowed Eastern to grow at a more measured pace and to invest significantly in research and development in technology.

He expects Eastern will continue to grow at the same pace as it has in the past, largely by buying smaller institutions about every two years.

Since Holbrook took over at Eastern in 2007, the bank has bought four institutions, expanding into New Hampshire and parts of Boston. Its assets have increased close to 40 percent from about $7 billion to $9.6 billion. Eastern has more then 100 branches.

Under his watch, the bank also launched a financial technology lab aimed at developing new tools for customers internally and using data analytics to sell more products to them.

Before he leaves, Eastern is also likely to cross the $10 billion in assets benchmark, which will trigger greater oversight from regulators.

Eastern already expects the additional expense of boosting its compliance team, upgrading its technology, and capping its interchange fee income when its cards are used at retailers will cost the bank $15 million annually before taxes.

The bank is preparing for the changes that come with reaching $10 billion in assets, Rivers said.

Deirdre Fernandes can be reached at deirdre.fernandes@globe.com. Follow her on Twitter @fernandesglobe.