Competing in the Novuna Great Britain and Northern Ireland team, the Colchester Harriers sprinter triumphed in the 200 metres, breaking the championship record in the process.
Hearn-Smith, 16, coached by Stephen Garnham at Harriers, clocked a superb time of 20.70 seconds in the final, which took place in hot and humid conditions.
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The Tiptree athlete had earlier run 20.89 in his semi-final, after posting a time of 21.37 in the heat.
A delighted Hearn-Smith said: “I’ve got no words, honestly.
“I didn’t think it would be that big a time.
“I was thinking shave 0.05 but no more, it’s more – I’m just so happy.
“So happy for everyone else who has come here – I’m over the moon and I’m so happy for everyone else.
“I thank my mum and dad, my sister, my coach – Steve and his wife Linda – they’ve been nothing but help to me.
“They’ve made me who I am as a character and an athlete so I just want to give all of my thanks to them.”
The European Athletics U18 Championships give young athletes the chance to showcase their talents against some of the best emerging athletes from across Europe.
Hearn-Smith recently impressed at the recent under-18 European Championship qualifying competition, clocking a personal best (PB) of 20.95 seconds.
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The Thurstable School pupil quickly became an internet sensation after uploading an Instagram video of him impressively winning his 100m race in under 11 seconds at a Schools Championship, last summer.
It was Hearn-Smith’s unique style of running that really caught the eye, leading to millions of views on Instagram, YouTube and TikTok and across wider social media.