The striker grabbed the third goal in a surprise 3-2 win over favourites Celtic at Hampden to ensure a return to the national stadium when Saints battle Hearts for the silverware on March 17.
Thompson has already won every major domestic honour in Scottish football from his time with Rangers.
But the Paisley-born forward says helping Saints claim their first trophy since 1987 would be extra special.
He told Press Association Sport: "The final is going to be a really proud moment for me and, if we win it, I guess it would go right to the top.
"To win a cup with St Mirren after a 25-year-wait, as a supporter, would go straight to the top of the list. But we need to win it first."
He added: "When I scored the goal, I think you could probably see on my face just what it meant to me personally.
"But I believe that winning semi-finals count for nothing, really.
"If we can repeat it and win the final, we would really be able to enjoy it more."