Montrose came from behind to earn a 2-1 victory in the first leg of the Championship play-offs semi-final at Links Park.
The second half belonged to the hosts. Russell McLean equalised in the 57th minute when he found space inside the box and curled inside the far post.
And Webster stole the show with 13 minutes left when he sent a powerful volley past the goalkeeper.
The other semi-final is even more finely poised after in-form Airdrie secured a 1-1 draw at Cove Rangers.
There were few clear-cut chances in an even first half and Airdrie took the lead 10 minutes after the break when Callum Gallagher's cross looped off Ross Graham and over the home goalkeeper.
Cove had a let-off when Dale Carrick missed an open goal from three yards after the ball flashed in front of him and Airdrie were soon pegged back in the 66th minute when former Diamonds striker Leighton McIntosh blasted home from close range.
Cove striker Rory McAllister forced a save and then hit the post before Kyle Connell came close for Airdrie late on.