United's owners, the Glazer family, listed the club on the New York Stock E in 2012 - but have retained a 90 per cent holding in the company.
Despite their problems on the pitch this season, United's share price has been slowly rising in recent weeks and Baron Capital's movement in the market could go some way to explain that increase.
In a securities and exchange commission filing on the United investor relations website, the club confirmed that Baron Capital now own 9,581,636 shares, worth a reported $151 million (£90.9m).
A further boost to the Old Trafford coffers has reportedly been sealed in the shape of a €700m kit deal with Nike - which Baron revealed had piqued their interest.
On the investment firm's website they provided information on the portfolio that includes the shares in United, and expressed confidence in their future prospects.
Their commentary read: "Shares of Manchester United dropped in Q4 (quarter four of 2013) due to a delay in the signing of a new global merchandise deal with Nike and the team’s poor performance on the field.
"The Nike deal is still expected to be signed, but has been pushed out from this fiscal year. We remain positive on the company’s prospects going forward."