John Mack, the former chief executive and chairman of Morgan Stanley, is joining private-equity giant KKR.

Mack will be a senior adviser to KKR, the company said in a release today. He'll be advising the group on its investments.

KKR co-founder Henry Kravis said in the statement that Mack was a "great leader who we believe will add enormous value to KKR's investors, partners and our portfolio. He will help make us smarter investors and strengthen our firm."

At the end of last year, Mack had stepped down from chairman of Morgan Stanley, the bank he had helped steer through the financial crisis after assuming the roles of chief executive and chairman in 2005 in the midst of a power struggle. He had given up the CEO role to James Gorman in 2010 but continued to help pitch the bank for big deals.

He remains a part-time adviser to Morgan Stanley and plans to continue in that role, according to a Morgan Stanley spokeswoman. He is also writing a book.

He had already joined one board of directors, as Rev Worldwide, a payment services company, said in November he would become a director.

"The volatile economic environment has created a demand for both capital and operational expertise and I am proud to join a team that offers both of those things and so many other resources that can help companies achieve their goals and investors meet or exceed their needs," Mack said in the release.

KKR describes its senior advisers as "current and former senior executives who have unique leadership skills that complement the work of the firm's investment professionals, including supporting new investing activities and providing counsel to KKR portfolio companies." The advisers often sit on boards of companies KKR has invested in.

Mack joins a group that includes Sir John Bond, former chairman of HSBC Holdings Inc.; Ford M. Fraker, a former U.S. Ambassador to Saudi Arabia; and James Owens, the former head of Caterpillar Inc.

Mack had first joined Morgan Stanley in 1972, though he took a brief four-year hiatus from 2001 to 2005, for part of it serving as co-CEO of Credit Suisse.

He was particularly known for rousing speeches to pump up his troops on the trading floor, and for pushing for risks in investment banking trades.

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