Astley opened up a commanding 4-1 lead and while Robertson's century in frame six suggested the tide may turn, Astley responded with a 119 break to move within one of a huge upset.
There was hope for Robertson when Astley broke down with the winning line in sight in frame eight, but Robertson twice ran out of position and would need to come out best in a safety battle to extend the match.
That did not appear likely when Astley potted the yellow to leave his opponent needing snookers, but Robertson secured one immediately, before asking Astley to play again with four colours remaining.
After some deliberation, Astley sent the brown around the angles and it dropped straight into the middle pocket, which he followed with an excellent mid-range blue to seal victory. It appeared an enormous fluke, but Neal Foulds suggested in commentary that it may have been a calculated risk - one which effectively ended the match.
It was the second high-profile success for an amateur, coming just says after Shaun Murphy complained that they 'shouldn't even be in the building' when he suffered a 6-5 defeat to Si Jiahui.
Robertson had earlier added his voice in support of Murphy, telling the BBC: "I totally get where Shaun's coming from. The amateurs are under a completely different pressure where there is actually no pressure, because they're not competing for ranking points.
"I completely agree with what Shaun's saying there."
However, upon defeat to Astley, Robertson tweeted: "He played a brilliant match and deserved the win today no doubt!"
He later told BBC Sport: "It hurts. As the defending champion you don't want to give up the title so easily, but you have to credit John.
"He played a brilliant match. He probably played the match of his life. There was a crowd, it was on the main table and he stood up to it and played as if he had nothing to lose."
