The Sky Blues' search for a record-extending sixth A-League Men championship ended in agony at Go Media Stadium, after Cam Howieson scored the lone goal in the 60th minute for Auckland FC.
Coached by Sydney FC great Steve Corica, Auckland became the first New Zealand team to win an ALM grand final.
It was Sydney's first appearance in a decider since the 2021 loss to Melbourne City - when Corica was in the Sydney dugout and Kisnorbo at the helm of City.
Grant, playing in his fifth grand final, admits Saturday's defeat left him with a strange mix of disappointment and hope.
Sydney's championship tilt had appeared unlikely when Kisnorbo joined in March as Ufuk Talay's replacement.
The grand final in front of 28,374 fans was Kisnorbo's seventh game in charge of the Sky Blues, and it was the first loss of his tenure.
Reaching the finale was already a historic feat, with Sydney becoming the first side to do so after finishing the regular season in fifth.
"When PK came in, we would have snapped a hand off him to be a final," Grant said.
"To be here and lose is disappointing, but I think there are a lot of positives to build on.
"It's obviously hard to see at the moment with the disappointment, but we know that we did really well when he came in.
"The proof is in the pudding."
Initially joining the club on an interim basis, Kisnorbo signed a three-year deal in the lead-up to the match.
He'll have plenty of young talent to work with, after handing grand final debuts to Akol Akon (17), Tiago Quintal (19), Wataru Kamijo (20) and Paul Okon Jr (21).
Akon, who celebrated his birthday on Thursday, became the youngest player to feature in an ALM grand final.
Sydney came to life in the dying stages, but couldn't find the equaliser to send the match into extra time.
"Even before the game, we knew that it didn't have to be pretty - you just want to get a result, don't you?" Grant said.
"I wouldn't say scrappy, but it felt like on a knife's edge there for a while.
"Both teams probably weren't playing the football that we wanted to, but it felt like if we got half a chance, we might just get one, but it wasn't to be."
AAP travelled to Auckland as a guest of Australian Professional Leagues.
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