The League Two side led 2-1 with 17 minutes to go before Posh turned it on to score three times.
County pulled a goal back deep in stoppage-time, but it was not enough to stop Posh claiming their place in the third round.
Our man David Lowndes took these snaps of just some of the fans who were there. Take a look and see who you know.
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1. Posh 4 Notts County 3
Peterborough United scored three late goals to turn the tie on its head and seal their FA Cup progress. Photo: David Lowndes
2. Posh 4 Notts County 3
Peterborough United scored three late goals to turn the tie on its head and seal their FA Cup progress. Photo: David Lowndes
3. Posh 4 Notts County 3
Peterborough United scored three late goals to turn the tie on its head and seal their FA Cup progress. Photo: David Lowndes
4. Posh 4 Notts County 3
Peterborough United scored three late goals to turn the tie on its head and seal their FA Cup progress. Photo: David Lowndes