Bury looked to have ended their six-match wait for a win when Damien Mozika slotted home Ryan Lowe's cross from the right in the 65th minute before he was promptly booked for over-celebrating by referee Pat Miller.
Southend striker Barry Corr spurned a great chance to equalise within a minute but shot over from eight yards with the goal at his mercy.
But French defender Mohsni, who was put on up front for Blues, saved the day for Southend when he nodded home Ryan Hall's cross from the right at the back post in the dying seconds of the five minutes of stoppage time.
Bury would have considered themselves unlucky not to have taken an earlier lead after top-scorer Lowe lobbed Blues goalkeeper Glenn Morris - but saw Chris Barker head his goalbound effort out before the striker latched onto a poor Peter Gilbert backpass and saw his shot rebound off the right post.
Corr's volley was comfortably saved by Cameron Belford before the goalkeeper pushed wide a 25-yard Anthony Grant effort.
Belford also denied Hall as Southend were left to wait for their first home win since October 30.
Southend 1-1 Bury: Match Report - view commentary, squad, and statictics of the game as it happened.