The investment by New York-based Baron represents about 5.8 percent of United's $2.6 billion market value but a dual-share structure means that the American Glazer family remains firmly in control of the club.
It is however a vote of confidence for United in a season in which the team has struggled on the field and risks missing out on a lucrative place in the Champions League for next season.
In its most recent investment guidance, Baron - describing itself as a long-term investor in growth businesses - said it remained positive on United's prospects, citing the possibility of a renewed kit supply deal with U.S. sportswear group Nike.
United shares closed at $15.84 in New York on Monday, up from a flotation price of $14 in August 2012.
Despite the listing, the Glazers maintain a tight grip on United through the ownership of a separate class of shares with 10 times the voting power of those traded on the stock market.- Reuters