Replacements: Paul Greenaway, Neil Simpson, Gareth Rourke, Ken McComb, Michael Scholes.
Replacements: Phil Whyte, Steven Sinnamon, Richard Holmes, Rowan Halsall, Glen Telford.
Playing against a very strong wind in the first half, the Tyrone side had only themselves to blame for being seven points behind at half-time.
The game opened with two penalties, one from Dungannon centre Stuart McCloskey on three minutes and an equaliser from Belfast Harlequins' goal-kicking scrum half Michael Heaney with 10 minutes on the clock..
The lively James Bates put Dungannon on the attack following a tap penalty, 'Quins cleared and countered with the Dungannon defence somehow stopping Michael Kirkwood from scoring.
Paul Magee and McCloskey then put 'Gannon on the offensive and hard-working hooker Paul Jackson kept popping up with the ball, keeping 'Quins on the back foot. However, a long clearance by the home side proved fruitful.
The ball went into touch close to the Dungannon line and following a quickly-taken throw-in, an over adventurous attempt to counter attack backfired and yielded a penalty for 'Quins.
Quick thinking from the Ulster-contracted Heaney saw that Dungannon were not in position following the efforts to keep 'Quins out, he punted the penalty across to the opposite flank for left winger Matthew Holmes to gather and score.
Heaney converted for a 10-3 scoreline. Dungannon had a great 30-metre drive stopped just short of the 'Quins line. They got a penalty outside the 22 but the attempt on goal was wide and the hosts took a seven-point buffer into the break.
Harlequins started the second half brightly and a promising attack was stopped by a doubletackle from Magee and Chris Cochrane.
'Quins kept on attacking but Dungannon defended aggressively and it was from this pressure defence that Dungannon scored.
A good tackle produced a sloppy pass by 'Quins, McCloskey swooped and gathered the ball. He broke past two defenders, the supporting Cochrane's presence was enough to distract the defenders and once McCloskey broke clear on a 75 metre-burst, 'Quins were unable to catch him.
Maybe when McCloskey lined up the conversion he regretted not going around behind the posts. Although he struck the ball well his kick went just wide.
Dungannon were held up as they quickly resumed on the attack, but a snap drop goal by Jaryd Bennett was successful to move the visitors 11-10 in front with 47 minutes gone.
The game ebbed and flowed with both sides creating pressure in their opponents' 22. Crucially, Dungannon struck for their second try in the 65th minute.
A good Dungannon scrum and great harrying by scrum half Bates led to a hurried cleared from 'Quins that his half-back partner Bennett charged down, collected and ran over for an unconverted try.
There was a degree of niggle in the game and seven minutes later, Dungannon were reduced to 14 men when replacement Glen Telford was sent to the sin-bin for retaliation following some foul play.
This put Dungannon on the defensive and Harlequins enjoyed a decent spell of pressure. The fact that Dungannon were six points ahead may have influenced Harlequins when two penalties were offered - needing a try they went for the lineout option.
Had they been eight points behind, they may have taken a penalty and who knows what might have happened. With a couple of minutes left another of 'Gannon's reserves, Richard Holmes, was sin-binned for slowing up the progress of the game.
Harlequins had a scrum on the five-metre line. The Dungannon scrum held out and when the scrum was reset, Johns' charges stood firm once again.
Eventually the final whistle went and a relieved Dungannon side could celebrate a good performance and a deserved win. With other results going their way on the day, 'Gannon are now clear of the relegation zone and can plan for next season.
Referee: Richard Kerr (IRFU)