Mairs & Power CEO Jon Theobald is retiring at the end of the year.

Succeeding him will be Mark Henneman, currently president and chief investment officer, the St. Paul-based investment management firm said Thursday.

"We've been in this transition for several months, if not years," Theobald said. "This just formalizes what were our plans for a long time."

Mairs & Power was founded in 1931 by George Mairs Jr. His son, George Mairs III, entered the firm in 1952 and was involved in active management until 2009.

Mairs III, who died in 2010, brought Theobald into the firm in 2002 to help ensure the long-term future of the firm. Theobald has been chairman and CEO of the firm since 2012.

Theobald said ownership of the firm was once concentrated in the hands of four people. Now ownership has been spread more broadly across the firm to 13 people. Spreading the ownership facilitated the firm's growth and helped lessen chances of being sold to private equity or other interests because the expense of buying out a retiring owner became too expensive.

"That was the vision of George Mairs III," Theobald said.

Since 2002, when Theobald joined the firm, employees grew from 13 to 43 and assets from about $1 billion to $9.6 billion.

Mairs & Power has three mutual funds and also manages private client funds for individuals, pension funds, endowments and foundations. About $6 billion of assets are in the firms Growth, Balanced and Small Cap mutual funds.

Henneman joined the firm in 2004 and has been the lead manager of the firm's flagship Mairs & Power Growth fund since 2013. He has been president and chief investment officer since 2015.

Henneman, who will continue to co-manage the Growth Fund, said by maintaining the firm's independence, Mairs & Power can continue to pursue its long-held investment focus.

"We will stick to the strategies that have worked so well over the years," Henneman said. "That's investing in companies in the Upper Midwest that have long-term growth and competitive advantages."

Also being promoted is Robert Mairs, grandson of the firm's founder. Currently chief compliance office, he will add the title of president.

Andrew Adams, who has been with the firm since 2006, will become chief investment officer. He has led the firm's Small Cap fund since its inception in 2011. Since 2015, he has co-managed the Growth Fund with Henneman.