Ryman League Hornchurch got a goal in each half to secure a shock defeat of their Third Division visitors. Both teams missed chances before Steve West put the non-Leaguers ahead with a penalty after Clark Keltie fouled him.
Barry Conlon missed a good chance to equalise and paid the price when Vinnie John doubled the lead after Jude Sterling's shot deflected to him.
Darlington's Gary Pearson and Ian Clark then both wasted late opportunities.
Hornchurch: Gay, Gooding, West, Adedeji, Kerrigan, Keeling, Locke, Sterling, Graham, Opara, John. Subs: Allen, Douglas, Southon, Rowland, Collier.
Darlington: Price, Clarke, Pearson, McGurk, Hughes, Keltie, Maddison, Nicholls, Morgan, Clark, Conlon. Subs: Bossy, Wainwright, Robson, Hutchinson, Norton.
Referee: A Penn (W Midlands).