Kah dominated South Australian racing in recent seasons before making the move across the border in January to fulfil her dream of winning a Group One.
That arrived on Saturday when $31 chance Harlem fought back to defeat Shillelagh ($18) by a short half-head with Trap For Fools ($15) a length away third.
“I'm speechless. That is just the best feeling in the world,” Kah said.
Now based at Cranbourne, Kah said she was prepared to travel to different training centres in a bid to build her career and connections.
“I didn't expect anything massive straight away so I tried to get to Euroa as much as I could and whenever they wanted me for jump-outs,” Kah said.
“And it's paid off.
“They supported me in Adelaide and asked when I was coming over here and make the leap.
“I knew it was going to be hard and I didn't think it would have so soon.”
Harlem's first Australian Cup win followed the second Newmarket Handicap win for stablemate Redkirk Warrior last year.
Redkirk Warrior was attempting a third victory in the sprint on Saturday, but after beating only one runner home he's been retired to Living Legends.
Despite his long price, Lindsay Park thought Harlem was the stable's best hope.
“He showed incredible fight, he was beaten twice and kicked back,” David Hayes said.
“I thought he was in terrific form, but I thought a few of them were.
“He goes well at Flemington and at the 2000 metres and it was a great ride by Jamie, an 11 out of 10 ride.”
Harlem will now head to Sydney where he”ll take on champion Winx in the Queen Elizabeth Stakes during The Championships.
“She's been beaten five times at Group One level,” Hayes said.
“She's a great filly and she stretched maybe the best colt seen in years.”
Corey Brown said the $1.65 favourite Avilius was a fraction disappointing after having a soft run.
“I hadn't asked him to really extend until I got the gap about the 300-metre mark and he didn't find a lot,” Brown said.
“He still whacked away well but I just expected him to put them to sword once he got through, but he didn't do that.”
Brown took the ride after Hugh Bowman stood down following his second-placed ride aboard $4.20 favourite Osborne Bulls in the Newmarket Handicap to return to Sydney for family reasons.