The former Barnsley and Bury full-back, who left Gigg Lane at the end of last season, put pen to paper on short-term contract this afternoon and could feature in the squad which visits Scunthorpe tomorrow.
Brown will remain with United until January when his future is scheduled to be reviewed by manager Chris Wilder and his coaching staff.
A decision is also expected shortly regarding fellow free agent Cameron Stewart who has also been training with the League One club’s first team squad.
Like Stewart, Brown, aged 24, turned professional with Manchester United before being capped by England at under-19 and under-20 level. He completed loans spells at Bradford City, Doncaster Rovers, Oldham, Coventry City and Ipswich Town ahead of a move to Watford in 2013. Brown joined Barnsley 12 months later before making 35 appearances for Bury last term.