The'Old Gold' came back from a two goal first half deficit to claim a Word Cup semi-final spot.
Dylan Blain and Jake Parry in his first start for the Steelmen, put the visitors comfortably ahead at the break.
Liam Thomas scored twice in the previous round at Bryntirion and two early strikes in the second half put his side level at Richmond Park.
Salt was rubbed into Port Talbot wounds as former favourite Liam McCreesh scored the winner for Carmarthen after 69 minutes.
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