The Giants led by nine points early in the final quarter of Sunday night's match on the back of a huge display from Clayton Oliver, who had 38 possessions and 12 clearances.
But two clutch goals from Jake Waterman and a towering mark up forward by Bailey Williams to kick his second major of the match helped West Coast secure the 13.10 (88) to 10.11 (71) win in front of 36,781 fans.
The result snapped West Coast's six-match losing run and improved their record to 3-7, already triple the amount of wins from last year.
GWS slipped to 4-6, with the unexpected loss dealing their finals hopes a significant blow.
Reid finished with 33 disposals, 10 clearances, 12 inside 50s, 10 score involvements, eight tackles, 788m gained and two goals in arguably the finest performance of his 49-game career.
The 21-year-old is the face of West Coast's future, and McQualter praised Reid for his performance and his leadership maturity.
"The sky's the limit for this young man, and he was exceptional tonight," McQualter said.
"First half, just really powerful. I loved his game, love how emotionally invested he is.
"Clearly he's a barometer for us. I think he gets the crowd going a lot as well, which is powerful, and plays his 50th game next week, so he's still at the very, very start of his career.
"He's got a long way to go still, and that's the exciting part for our fans and our club, that he's actually got a lot of scope still to be a better player."
Williams, who was overlooked for the ruck role in the early part of the season in favour of Matt Flynn, also produced a career-best display with 40 hit-outs, 16 possessions, nine tackles, seven clearances and two goals.
"We know how athletic (he is), and he goes forward and takes a few clunks," McQualter said.
"I think he tried to Jackie Chan one out of mid-air there at one time. We might put that one away, Bailey.
"But I thought it was a really powerful performance and helped our midfield compete really hard."
West Coast defender Reuben Ginbey kept GWS spearhead Aaron Cadman to just 0.1 from five disposals in another important effort from the undersized key backman.
Oliver tallied 12 disposals and five clearances in the opening term alone but GWS were only ahead by 10 points at quarter-time on the back of some shoddy 2.6 goalkicking.
The second quarter was the Harley Reid show as the No.1 draft pick from 2023 tore the game apart.
Reid tallied 14 disposals and kicked two goals for the quarter - including a clutch set shot from near the boundary - as the Eagles kicked six of the first seven goals of the term.
GWS turned the tide in the third, but West Coast regrouped in the last to secure a much-needed win.
Giants coach Adam Kingsley says he has some short-term concerns surrounding player availability and inconsistent form, but is confident his team will find a way to turn around their fortunes.
"I'm still confident in this group," Kingsley said.
"I think we've got a really strong group. I think the way that we play is strong. We need to be more consistent in that."
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