Hitchon is aiming to represent Britain at the 2012 Olympics
Blackburn Harriers pair Sophie Hitchon and Holly Bleasdale claimed gold and bronze respectively at the World Junior Championships in Moncton, Canada.
Hitchon threw 66.01 metres in the hammer to better her own UK junior record and beat Slovenia's Barbara Spoiler, whose best effort was 65.28m.
"I knew I just had to go for it on my final throw," reflected Hitchon after her last-gasp winning effort.
Pole vaulter Bleasdale cleared 4.15m to finish third.
"It's amazing, I wasn't sure I'd get a medal," said Bleasdale, who failed with attempts at 4.20m and 4.25m.
Hitchon won bronze at the 2009 European Juniors Championships and is second behind Zoe Derham in the UK senior hammer rankings.
The 19-year-old's previous UK junior best stood at 65.98m and she claimed the tight nature of the competition had spurred her on to set a new personal best.
"The fact that I was neck and neck with the Slovenian all the way through made it a really good competition and really drove me forward," added Hitchon.
There was disappointment for Briton Lawrence Okoye in the men's discus.
Despite being ranked third in the world, the Croydon Harrier had to settle for sixth with a best throw of 59.77m.
"I'm really upset," he said. "My last throw felt good but I smashed the cage. I didn't connect with any of my throws which is appalling - you can't come to a major championships and not connect."