Stenning brings more than 30 years' experience to the firm and will assist with Downing's growth.
He will also launch the Downing Active Defined Benefit fund, according to a Companies House filing.
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Stenning joined Atlantic House in 2019 as a non-executive director, before taking on the chief executive role in July 2020.
He also served as head of BlackRock's UK retail business, holding roles with Mercury Asset Management and Merrill Lynch Investment Managers, which merged with BlackRock in 2006.
When he departed the firm in 2016, he took on the role of deputy chair, and then chair, of The Investing and Saving Alliance.
In 2019, during the fallout of the collapse of the former Woodford Equity Income fund, Stenning took over as chair of Link Fund Solutions, authorised corporate director to the fund.
He was quietly removed as a director in February 2021, with the chair position taken up by Alistair Reid, former CEO of CRUX Asset Management.
Stenning will be joined by Huw Price, who joins as Downing's head of alternatives structuring, also bringing more than 30 years' financial services experience to the role.
Most recently, Price was an executive director at Santander Asset Management, responsible for transformation and strategic change.
Stenning said: "I am thrilled to be joining Downing at this exciting time of growth for the firm.
"Downing has built an impressive platform and reputation over the last decade for spotting opportunities and designing products that truly meet investor needs. I look forward to working with Huw and the team here to help the firm expand and offer additional opportunities to our investor base."
Kostas Manolis, head of private market investments, added: "We are delighted to welcome Tony and Huw to the firm. Tony's significant public markets experience and his deep knowledge and creativity make him the ideal person to bolster Downing's strategic growth, complemented well by Huw's significant expertise.
"We look forward to the valuable contribution both will bring as we continue to expand our capabilities in line with client demand."