McDermott, who missed two matches after a training mishap, top-scored with an unbeaten 53 from 34 deliveries as his side chased down 8-150 with 11 balls to spare at Blundstone Arena on Monday night.
The victory, Hobart's second of the season from five matches, jumps them to fifth spot on the ladder and two competition points outside the top four.
McDermott shared an 85-run partnership with the in-form Mac Wright, who fell late in the chase for 34 from 25 balls.
Wright's knock came on the back of an impressive 36-ball 63 against the Renegades and 33 from 19 against the Stars.
The pair took 22 runs from the two-over power surge taken in the 13th over to get the chase down to roughly a run a ball.
The Thunder are long odds to make the finals, with just one win from six starts in a shortened 10-games-per-team competition.
Earlier, skipper Chris Green gave the Thunder's innings a late boost with an unbeaten 33 from just 17 deliveries after coming to the crease at 6-92.
Sydney lost regular wickets throughout, including English import Tom Kohler-Cadmore, who was run out without facing a ball.
Kohler-Cadmore, who has a career strike-rate of 140 at an average of almost 30, unsuccessfully took on the arm of Wright in the deep in the fourth over.
Chris Jordan, Nikhil Chaudhary and Paddy Dooley shared the spoils for the Hurricanes with two wickets each.
Veteran seamer Jordan picked up an economical 2-20 from four overs, including the dangerous Oliver Davies (20 off 17) during the power surge.
Chaudhary (2-26 from four) had opener Cameron Bancroft (21 from 20) caught and bowled with his spin, then returned in the 15th over to trap Nathan McAndrew lbw.
Caleb Jewell (31 from 20) got the Hurricanes' chase off to a strong start but was out in the seventh over to a low outfield catch by Daniel Sams, ruled by the third umpire to have been held just above the grass.
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