Andy Wardell opened the scoring for United after 20 minutes of a hard-fought match with a great finish from the edge of the penalty area.
Seamer battled back strongly in the second half, hitting the woodwork through efforts from Gaz Lawton and Rob Speight.
United doubled their lead with a breakaway goal from Darren Clough, the striker beating the offside trap before rounding the Seamer keeper.
Most Popular
-
1
Scarborough Athletic's home National League North campaign off to a winning start against Hereford
-
2
RETRO SPOTLIGHT: 15 nostalgic Scarborough photos from the Scarborough News sports archives
-
3
Ebberston battle back to sink hosts Filey in SBL Premier Division thriller
-
4
Scarborough Athletic slip to 2-0 defeat at Brackley Town in National League North opener
-
5
Commonwealth Games 2022: Scarborough's Chris Turnbull and Team England win bowls mixed para pairs bronze medal
Danny Glendinning pulled a goal back for Seamer from the penalty spot.
Seamer were then reduced to 10 men after Connor Myerscough was sent off, allowing United to hold on to make their first League Cup final since 2007.
Filey Town take on West Pier in the second semi-final on Wednesday March 27, 7.30pm kick-off, also at the FLS.
