Both sides had early possession but it was Langford who got the early breakthrough when Meshack Hendrickson burst into the penalty area and was pulled back by a defender.
Michael Moss stepped up to slot home his 10th goal and ninth penalty of the season.
Harpenden fought their way back into the game and tried to play good passing football, but the pitch upset the rhythm of both sets of players. Town piled on the pressure but Nick Gardner in the Langford goal kept them at bay.
Then after 35 minutes Francis Jowie’s corner was met unopposed by Jake Cartright who’s powerful header found the back of the net.
Two minutes later Francis Jowie had a fierce shot defected into the Reds net, putting the home team in front.
The second half saw continued Harpenden pressure but they could not finish. Langford were still very much in the game, and gradually started to keep more possession, even having a strong penalty appeal turned down.
With both teams looking weary Harpenden broke away and Sam James smashed home Harpenden’s third. Langford were unable to find a reply in the remaining five minutes.
Langford host Hillingdon Borough in SSML Division One on Saturday, before welcoming Wembley in the League Challenge Trophy on Tuesday.
Langford: Nick Gardner, Kai Ashley, Deon Putnam, Steven Redmond, Craig Meyrick (capt), Michael Moss, Meshack Hendrickson, Sean Murray, Tyler Ingham, Jason Penman, Stephen Brooks.
Subs: Lewis Ellis (used), James Chisholm (used), James Potter (used), Ross Scotney, Karl Gudgin.