The $13 billion Chicago-based money manager, which has grown into a beacon of opportunities for diverse talent in the asset management industry, also moved a step closer to an eventual ownership transition as Ms. Hobson, 50, acquired enough shares to make her the new majority owner, with 39.5% of the business.
But John Rogers, 61, the co-CEO, chairman and chief investment officer who founded the firm 36 years ago at age 24, made it clear he is still at least 14 years from retirement.
"My hope is, I'd love to be here when we celebrate our 50th anniversary," he said. "I understand the importance of planning for the future and making sure there's a succession plan in place, but I plan on doing this into my 70s, and I'm not going anywhere or planning to do anything differently."
By selling a 14% stake in the business to Ms. Hobson, Mr. Rogers' ownership drops to 34.1%, while employees and board members own 21.8%, and the remaining 4.6% is owned by outsiders.
Mr. Rogers, who has always focused more on the asset management side of the business, including the $2.2 billion flagship Ariel Fund (ARGFX), described Ms. Hobson as a "seven-day-a-week type of person" who has overseen general business operations as company president for the past 19 years.
Ms. Hobson, who was unavailable for comment for this story, joined Ariel 28 years ago as an intern.
In addition to her work at Ariel, she serves on the boards of Starbucks, JPMorgan Chase and Quibi, a short-form video content company.
Earlier this year, Ms. Hobson was awarded Princeton University's highest honor, the Woodrow Wilson Award, presented annually to the alumnus whose career embodies a commitment to national service.
Megan Carpenter, co-founder and CEO of FiComm Partners, credited Ms. Hobson and Mr. Rogers with "leading by example and demonstrating the success that comes from an honest belief in fairness and equality."
"This appointment and share purchase are not a 'women in leadership' strategy for Ariel Investments, this is a fundamental operational belief in the power of diversity and inclusion," Ms. Carpenter said.
Tina Powell, CEO of C-Suite Social Media, also celebrated the high-profile promotion.
"I've long admired the career path and trajectory of Mellody Hobson," Ms. Powell said. "It provides evidence and confirms the idea that female leaders are a vital part of a thriving enterprise."
Ms. Hobson's elevated profile is not lost on Mr. Rogers.
"Mellody has, over the years, already taken control of all the non-investment areas of the firm, and it's a natural evolution for her to become a co-CEO and show clients and prospects that we are partners," he said. "She really is a role model for talent, like a Pied Piper for talent, and we're in a growth mode and she's building out some added talent to make sure we stay competitive in a competitive marketplace."
While Ms. Hobson is technically based at Ariel's Chicago headquarters, Mr. Rogers said she also splits her time between the firm's New York and San Francisco offices.
"I would say she lives in New York, Chicago and San Francisco," he said. "She works extraordinarily hard seven days a week, which includes a standard Sunday call."
While the immediate impact of the promotion is more about recognizing and rewarding Ms. Hobson's hard work and dedication, the step toward ownership transition also puts Ariel ahead of a lot of business owners in the financial services industry, said Carolyn McClanahan, founder and director of financial planning at Life Planning Partners.
"I think it is fantastic and applaud John Rogers on this move and the fact that he has thoughtfully done what most business owners fail to do: create a good succession plan and actually carry it out," she said.