Benari joined Challenger in October 2008 and is responsible for the group's business services division.
"[Benari] is a pragmatic business leader with broad experience, having been group CFO and COO for the last three years, as well as chief executive of our highly-profitable mortgage management division for more than five years.
In fact, Brian was the driving force behind the acquisition and expansion of our residential lending," Challenger chairman Peter Polson said.
Stevens will continue as chief executive until the company's February 2012 board meeting, and will remain at Challenger until the end of the 2012 fiscal year to ensure there is a smooth transition of responsibilities.
Meanwhile, Challenger has appointed Cathy Hales as its new general manager of boutique partnerships.
Hales joins Challenger from institutional asset management specialist RREEF in New York, where she was chief operating officer of client relations.
She has also previously worked across the funds and wealth management industry at Deutsche Asset Management, Colonial First State and BT Financial Group.
"We are delighted to appoint Cathy to the role. I'm confident she'll be a valuable asset for our existing partners as well as supporting the business's long-term growth aspirations," Challenger Funds Management chief executive Rob Woods said.
