A surging Stefan Law drive into the box for the Viewpark team ended with Daryl Meikle scuffing an effort agonisingly wide.
The young midfielder atoned for this however in the 14th minute when finding the bottom corner with aplomb from 12 yards after a ball in the box had been won well by Brendan Smith.
Played on Keanie Park’s vast expanses, finding space was never going to be an issue and even more so in the 25th minute when the hosts were reduced to 10 men after centre back Gregg McBride received a second yellow for a reckless challenge on United debutant Ali Morgan. As is often the case in these situations it was the numerically burdened side who then scored.
A free-kick from halfway by Stephen McGarrigle found Steven Fitzpatrick who steered an effort low past ‘Wood keeper Derek Barnes.
Thorniewood regrouped and restored their lead two minutes later when a long Johnny Logan throw was knocked back by Craig Forbes for Sinclair to dispatch home from six yards.
Now in control, United had a few half chances before on the stroke of half-time a Damon Welsh effort flew inches wide to end a decent first half showing.
Beginning the second half in search of an all important third goal, ‘Wood continued to impose with Law passing up a real opportunity in the 50th minute when firing over from the edge of the box.
‘Burgh had another setback in the 52nd minute when goalkeeper Robbie Kelly had to be replaced by back-up David Frances Brolly.
Showing they weren’t totally out of things Johnstone’s Chris Docherty had Barnes saving low to his right as he tested him with a 25 yard daisy cutter before the ‘Wood number one had to tip over a speculative effort from distance.
Johnstone Burgh were then awarded a soft looking penalty in the 69th minute. However Barnes proved up to the task, guessing correctly to save Fitzpatricks attempt.
Looking a bit ragged for the first period of the match, United could only watch in frustration as Law lost control as he was about to round the keeper in the 67th minute before celebrating the crucial third in the 71st minute when a fierce Sinclair shot was blocked by Brolly with Meikle following up to head home from close range.
With the insurance of a two goal advantage United were now looking comfortable again and pushing hard for a fourth.
This didn’t quite materialise with the best chance in the closing stages being a Welsh header flying just wide after he was found by a quality Keigan Parker cross.
At the other end, Morgan did well to clear a last minute Liam McGonigle effort off the line.
Another important win that sees Thorniewood rise to eighth with a number of vital games in hand.
Thorniewood United: Barnes, Watson, Morgan, B Smith (Jackson 61), Ferguson, Welsh, Forbes (Jack 85), Meikle, Logan (Parker 68), Law, Sinclair. Not used: R Smith.