The 28-year-old, from Gravesend, was working at the Bluewater branch of House of Fraser when he got a call that his pregnant partner Becki Harman was in pain.
Rushing home to Venture Court to help care for their two-year-old son Harry, he found her in the bathroom.
Mr Dempster rang the midwife, who instructed him to get some hot towels and lay cushions on the bathroom floor.
The couple were assured an ambulance was on its way, but the baby refused to wait – and Mr Dempster suddenly found he was delivering his son.
Nine minutes later, at 12.30pm on a Saturday afternoon, Freddie Daniel Dempster entered the world.
Mr Dempster said he did not have time to be nervous or scared and just focused on delivering their baby.
He said: "It was such a short period of time. It was nine minutes from me looking at the head coming out to the end.
"I didn't have time to think - it was only afterwards I realised what I had done.
"I was on the phone to the midwife the whole time, the phone was resting on my shoulder.
"Becki was in a lot of pain, she had no pain relief.
"She was saying 'the ambulance needs to get here – bring the gas and air'. It was hard, but I had to say the gas and air isn't coming."
Paramedics from Medway Maritime Hospital arrived 30 minutes after Freddie was born and checked both mother and baby were well.
He said family and friends did not believe him when he told them the news, but nothing could beat the feeling of being a dad and delivering his son is a memory he will cherish.
He said: "It's a bit of a cliche, but there really isn't anything better and it's a story that's going to go on forever. It's incredible."
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