Jay May scored the only goal as Ashford reached the Isthmian South East play-off final.
The veteran striker’s early tap-in proved enough to see off a Cray Valley side whose second-half dominance went without reward.
United will host Herne Bay, who beat Haywards Heath on penalties in the other semi-final, for a place in the Isthmian Premier on Saturday.
For all the Millers’ possession in that second half, a disciplined defensive display from the hosts, led by the outstanding Liam Friend, meant they didn’t work keeper Jordan Carey until added time.
Ashford made the perfect start with a well-worked opener in just the sixth minute.
Tommie Fagg, bright throughout the first half, latched on to a Gary Lockyer flick, broke into the box and squared for May, who couldn’t miss.
The Nuts & Bolts, enjoying home advantage after finishing runners-up in the league, mixed up their play well early on, playing some nice stuff through midfield, while also hitting May, who barely lost a header.
Cray Valley grew into it, with Marcel Barrington and wideman Denzel Gayle keeping the defence on their toes.
Indeed, Gayle hit the base of the post after a neat move involving Barrington, Anthony Edgar and Connor Dymond.
Frannie Collin thumped a free-kick out of the ground while another set-piece presented Josh Wisson with a decent chance but he couldn’t connect cleanly.
At the other end, Carey pushed Nathan Green’s low drive out for a corner and Edgar fired wide after working space for a shot shortly after the half-hour mark.
Cray Valley made a good start to the second half, looking sharp in possession and forcing Ashford back, without really creating a chance as the hosts dug in.
Ashford introduced the midfield bite of Jarred Trespaderne, who was soon getting about the pitch, but it was largely attack v defence.
Thr Nuts & Bolts backline were proving up to the task, protecting Carey’s goal as the clock ticked down.
The Millers finally worked Carey in injury time but even then it was a comfortable save to deny Gayle from the edge of the box.
Lockyer could have settled it moments later but fired over, while Green hit the post for Cray at the death.
Ashford: Carey, Simms, Diallo (Ter Horst 87mins), Wisson, Friend, Kamara, Collin, Fagg, May (Burdon 77mins), Lockyer, Kwayie (Trespaderne 62mins). Subs not used: Hook, Steventon.
Cray Valley: Lewington, Dymond, Green, Tumkaya (Babalola 83mins), D Gayle, Bartington, Brefo, Warren, Edgar, Ibrahiym (Adeyemo 71mins), I Gayle. Subs not used: Semakula, Blanks, Hill.
Referee: Kane Dempster.