Campbell defeated Ireland's John Joe Nevin 14-11 in a tense bantamweight final to add to the historic gold medal claimed by teammate Nicola Adams and become the first Brit to win at this level since 1908.
And the 24-year-old from Hull struggled to describe his feelings after seeing his hard work rewarded with a deserved gold medal, but admitted he couldn't have predicted how the final with his familiar foe would turn out.
"I'm lost for words. I'm very emotional and it's something I've worked for all my life and I can't believe it's here - I really can't," he said.
"You can look at it both ways - he knew me as well as I knew him, so we both knew each other. He's a top fighter. It was a very difficult fight - very technical - and it was a case of outwitting each other.
"He's a strong fighter and at punching, and I had to be aware of that. I had to be on my game to win and that's what happened - I was on my game tonight."
The ExCel was the stage for the first ever Olympic final to feature an Irish boxer against a British boxer and it was difficult to determine who the home representative was, such was the support for both.
With both men having faced each other twice before and won one each, it was always likely to be an even bout but it was Campbell who took the first round 5-3.
But Nevin came back stronger in the second round which he edged to take the scores to 9-8 in favour of the Brit.
Campbell continued his fine display in the final round to force Nevin into a standing count as the former European Championship gold medallist secured the win in style.
- John Joe Nevin