Coming into the final ranked third after the short program the Aussie couple were all class, outstripping their rivals from the world's skating powerhouse nations to claim the gold medal ahead of Aleksandra Boikova and Dimitrii Kozlovskii from Russia with China's Yumeng Gao and Zhong Xie taking bronze.
It follows their ground-breaking gold medal at a Junior Grand Prix in Estonia last year, another first for Australian figure skating.
Indigenous skater Harley, from Western Sydney, and his Moscow-born partner Katia scored a personal best for their free skate of 104.16 for a final score of 163.98 - another personal best despite a fall on their throw salchow.
They pulled off a clean throw triple flip and side by side triple jump combos in what was their skate of the season.
"I'm shocked that we got first place, it's crazy to put Australia in the scene for skating and we're over the moon about the gold medal, I can't put it into words," Harley said.
An ecstatic Katia added: "I can't believe it".
Hearing the Australian anthem played at a Junior World Figure Skating championships was moment the pair, and a proud country, will savour for ever.
"Mum and dad this is for you!" Harley said from the kiss and cry as they waited for their score.
There were high hopes for this charismatic team going into the Junior Worlds and they didn't disappoint. After placing third in the short program, the Australians were first to skate in the final group of four pairs and their shot at a medal.
Their 4.30 minute free program was packed with difficulty and they delivered with confidence, opening with a quality split triple twist, following up with two side-by-side triple jumps and then the throw triple flip. With mounting points already on the board, the pair relaxed into an engaging performance that ultimately couldn’t be beaten.
Watching in her Sydney home as her husband Andrei travels with their star students, co-coach Galina Pachin was overwhelmed.
“This is a big thing. I am crying and just so proud,” she said.
The Pachins have taught Windsor since he was a nine-year-old and found a partner for their young charge in Moscow when all attempts at a local partnership failed. It was instant magic when Windsor and Alexandrovskaya first teamed up in Sydney in January 2016 and began competing for Australia mid last year.
The pair split their training between Canterbury Ice Rink in Sydney and Moscow with their Sydney-based coaches Andrei and Galina Pachin and will return to Moscow for less than two weeks training before competing at the World Championships in Helsinki, Finland to vie for an Olympic Qualification spot for Australia in the 2018 PyeongChang Olympic Winter Games next February.