Led by a five-goal haul from the mercurial Jake Stringer and a dynamic running midfield, the Giants brushed off their mounting injury list to claim a 19.8 (122) to 14.11 (95) victory at Engie Stadium.
"Often happens in this game but we got what we deserved," Mitchell said.
"I thought they outworked us from the very start of the game.
"And our numbers around halftime, I think we gave up 35 marks in the second quarter and we just weren't able to slow them down and get the game on our terms at all in that second quarter in particular, but really the majority of the first half.
"They made the most of their chances going forward and took the game away from us pretty quickly, really."
Mitchell's Hawks, who failed to snag Essendon gun Zach Merrett and are without the injured Will Day, were outclassed in the midfield.
The Giants' hard running, led by Finn Callaghan (26 disposals, 735 metres gained) and Lachie Whitfield (27 disposals) was notable in a bitter defeat for Hawthorn.
"We have no excuses to put in that sort of workrate," Mitchell said.
"Everyone comes into this time of year unsure and we thought we were much more prepared and we'd be much better equipped to handle what they produced than we were."
Mitchell was adamant Hawthorn could rectify their issues ahead of facing Merrett's Bombers next Friday.
Hollywood superstar Tom Hanks, in Sydney shooting a movie, was among those in the crowd as Stringer put on a show.
"He's working hard on and off the field in terms of he trains with great purpose, he trains with great intensity but he also studies film a lot, he's dragging others along with him," Kingsley said of the dynamic forward.
With Tom Green out for the season and Aaron Cadman, Sam Taylor and Brent Daniels sidelined, recruit Clayton Oliver (27 disposals, nine clearances) and veteran Stephen Coniglio (two goals, 22 disposals) were influential.
Oliver, unwanted at Melbourne last year, also kicked a sensational goal in the third term.
"He's so clean. Sometimes he's too fast with his hands in terms of I'm not sure our players are expecting that ...it felt like it caught a few of us by surprise at times," Kingsley said.
"But they'll start to gel together even more the more they play together and Clayton's impact will just continue to increase."
Young defender Joe Fonti (22 disposals, 10 intercepts) and "outstanding" ex-Bomber Jayden Laverde were excellent.
Hawthorn co-captains James Sicily (23 disposals, 14 intercepts) and Jai Newcombe (25 disposals) battled hard while Nick Watson (two goals) and Jack Gunston (four goals) had moments.
The turf appeared patchy after the ground recently hosted two Bad Bunny concerts but players didn't have any issues with the surface.
GWS led by 19 at quarter-time after a high-scoring opening term, courtesy of Jesse Hogan's three majors.
Hawthorn trailed by 39 at both halftime and three-quarter time.
The Hawks mounted a late fightback, cutting the deficit to 22 before GWS debutant Phoenix Gothard kicked a sensational goal to slam the door shut.
GWS next face the Western Bulldogs next Saturday.
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