Gary Sheahan was left unmarked six yards out on 40 minutes as goalkeeper Darren Quigley's parry from Richie Fitzgerald's angled drive broke directly into his path.
Fingal knew beforehand that only three points would suffice for them to go above UCD and Shelbourne at the summit on goal difference but they lacked the fluency so evident in their previous stretch of ten games unbeaten.
They did make a fist of it in the second half and Wexford had Packie Holden to thank for a producing a number of important saves.
Fingal skipper Shaun Maher failed to get his header from Shaun Williams corner on target after 66 minutes but the home side's best chance was squandered by Gary O'Neill seven minutes later when he fired straight at Holden despite getting a clear sight on goal.
Midfielder Robert Bayly blasted over from ten yards two minutes later as the night of woe continued for the title chasers.
Wexford pressed for a second and might have found it eight minutes from time when Shane Dempsey's header came back off the post with Quigley stranded.
But the Youths held out for the win, continuing their record of beating the four teams above them in the Division on their travels.
Sporting Fingal (4-3-3): Quigley; Gannon, Maher, P. Byrne, Frost; Bayly, McFaul (Dawson 79), Williams; C. Byrne (Zayed 61), O'Neill, Kirby (Whelan 61).
Wexford Youths (4-4-2): Holden; Russell, Breen, Malone, McCurtin; Sinnott, White, Dempsey, Fitzgerald (Aust 81); Sheahan, Keohane.
Referee: T. Connolly (Dublin).
Attendance: 323.