It took little more than 20 seconds for King to fire United ahead, pouncing on a slapstick defensive collision to calmly slot home the opening goal of the game.
The Pars had a mountain to climb 10 minutes later. Magnus Eikrem's measured pass found Ajose inside the area, and the livewire forward curled a perfect finish just inside goalkeeper Chris Smith's post.
Although their side were being run ragged, the home faithful generously applauded United's next goal - a superbly powerful shot from King on 16 minutes. Possebon added a fourth, also from distance, which gave Smith no chance.
Ole Gunnar Solskjaer and Warren Joyce rang the changes at half-time and throughout the second half, and the Pars pulled a goal back through Andy Kirk's free-kick.
The final word went to United, however, when Keane latched onto a fine pass from Robbie Brady and fired a fabulous finish just inside substitute goalkeeper Kyle Allison's post.
United: Devlin (Johnstone 46); Wootton, Chester (Tunnicliffe 64), Gill, Dudgeon (Fryers 64); Norwood (Brady 46), Possebon, Eikrem, Morrison (Stewart 46); King (W Keane 46), Ajose (Lingard 64).