The Frenchman latched onto a loose ball and fired home from close range past grounded Crewe keeper Steve Phillips after 74 minutes to keep the Shrimpers top of the table.
Phillips had done well to parry a thunderous 25-yard free-kick from Southend substitute Ryan Hall following a foul on Michael Timlin, but had no chance with Mohsni's follow-up.
The match had looked set to be a stalemate after Phillips had earlier denied Mohsni and Liam Dickinson saw his effort cleared off the line by Carl Martin.
At the other end, Southend goalkeeper Glenn Morris pulled off a fine triple save to keep out the Railwaymen.
Luke Murphy's shot from just outside the box was parried by Morris, but the keeper was unable to keep hold of the ball and had to be alert to stop Shaun Miller's attempt at his left-hand post before blocking Greg Pearson's follow-up.
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