Russ Penn's side grabbed the only goal of the play-off eliminator less than a minute into the clash.
It wasn't a game that will live long in the memory with a lack of chances created by both sides - but it was the visitors who managed to hold on to book a place in Sunday's last four clash with Kings Lynn Town.
Harriers were in dreamland after just 33 seconds as a cross from the left was met by Amari Morgan-Smith whose headed effort nestled in the corner.
Chances were few and far between in the first period after the early goal - but Kidderminster controlled the game and went in ahead at the break.
The second half remained a cagey affair with chances at a real premium - before Alfreton were forced to hook an effort from a corner off the line to keep it at 1-0.
Ashley Hemmings, who bagged in the final day race for the play-offs curled a late effort just wide but as it turned out one goal was enough with Penn's side now 180 minutes from the National League.
