Alfreton secured their third win in four after condemning Forest Green to their fifth consecutive defeat.
Josh Law's drilled effort gave the hosts the lead following good work from Alastair Taylor on nine minutes.
Taylor then forced away keeper Sam Russell into a fine save, before Nathan Arnold doubled the Reds' advantage with a fine strike before the break.
Jamie Turley nodded in James Norwood's cross to reduce the deficit, but they could not level.
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Forest Green Rovers Manager Dave Hockaday told BBC Radio Gloucestershire:
"The first half performance was awful, the second half we were very good and should have won the game twice over.
"It's not about anything other than looking at ourselves to get stronger mentally. They've got to ask themselves why they were so poor in the first half, and then utterly dominated the second half.
"In many respects we've got to be glad that the season is over because we have limped over the finish line. There's a lot of hard work and thinking to do.
"Up until the last four games, we were in with a chance of the play-offs, but we haven't proved to be mentally strong enough."