Fidelity Investments said Ronald P. O’Hanley, the firm’s head of asset management since 2010, will leave at the end of February, without giving a reason.

The Boston-based firm, the second-largest mutual fund company, plans to replace O’Hanley with an internal successor, whom it didn’t name, according to a memo to employees today from President Abigail Johnson.

“Ron has effectively led the globalization of our investment team, driven solid investment performance across asset classes, built influential stakeholder relationships and launched innovative advancements to existing and new products,” Johnson said in the memo.

O’Hanley, 56, took over as Fidelity was struggling to recover from the 2008 financial crisis and investors fled actively managed stock funds, the firm’s traditional area of strength. Two years later, the company named Abigail Johnson president, positioning her to succeed Chairman Edward C. “Ned” Johnson III.

O’Hanley is the second high-profile executive to quit at a top U.S. money manager in as many days, after the resignation of Pimco head Mohamed El-Erian yesterday.

‘Family Company’

“Because it is a family company, the number two job at Fidelity is a challenging one,” Russel Kinnel, director of mutual-fund research at Chicago-based Morningstar Inc., said in a telephone interview. “Historically it is a position in which people don’t stay very long.”

O’Hanley joined from Bank of New York Mellon Corp., where he also oversaw money management, as Fidelity split its investing and distribution businesses under two presidents. Abigail Johnson simultaneously took over all client-facing units the same day.

At the end of 2010 Fidelity managed $1.59 trillion, according to the company. While that amount rose to $1.7 trillion as of Oct. 31, Fidelity lost ground to faster-growing rivals including New York-based BlackRock Inc. and Vanguard Group Inc. in Valley Forge, Pennsylvania.

Fidelity’s market share of the U.S. mutual funds business fell to 10.5 percent at the end of 2013 from 10.8 percent in 2010, according to data compiled by Morningstar.

‘Tough Time’

“It has been a tough time for Fidelity,” Geoff Bobroff, a mutual-fund consultant based in East Greenwich, Rhode Island, said in a telephone interview. “They have lost market share and when that happens the guy on top usually gets blamed.”

O’Hanley arrived at Fidelity with the goals of improving asset management and helping Abigail Johnson take over the company from her father, according to John Bonnanzio, editor of Fidelity Monitor & Insight, a newsletter for investors, based in Wellesley, Massachusetts. He feels he accomplished both, said Bonnanzio, who spoke to O’Hanley today.

“On reflection, the time is right for me to move on,” O’Hanley said in a separate memo to employees today. He said he plans to spend more time with his family and nonprofit organizations, before pursuing a new professional challenge.

While O’Hanley struggled to attract assets, he pushed Fidelity into new product areas such as exchange-traded funds. The firm opened 10 equity ETFs in October, each focusing on a broad industry group. It also filed for permission from regulators to introduce actively managed ETFs.

Given Abigail Johnson’s still young tenure as president of all of Fidelity’s main businesses, investors should watch closely who will replace O’Hanley, James Lowell, editor of Fidelity Investor, a newsletter based in Needham, Massachusetts, said in a telephone interview.

“It is imperative that Abigail has a trusted wingman going forward,” he said. “That is critical for running the company.”