Mum-of-two Natalie Whiteside, 23, said her youngest child Kian started to choke on his own vomit while in a Sainsbury's supermarket.
Whiteside claims she called out to two Sainsbury's employees for help, but they ignored her and just walked past, before a stranger intervened to help save Kian's life.
"This man stopped and asked if I needed an ambulance and I said I didn't know what was wrong with him [Kian]," said Whiteside.
"The man took Kian and sat on the floor holding him and rubbing his back."
Whiteside's son Kian
The incident happened at Whiteside's local branch of the supermarket in Lancaster on 14 September.
Whiteside said after she put Kian in a trolley seat, he vomited and started gasping for air, before stopping breathing and turning grey.
She added: "It was a good two minutes before Kian caught his breath.
"I've had first aid training but I just panicked and froze, I didn't know what to do - he changed colour so quickly.
"If that man didn't come over I could have been sat on the floor forever. I was so grateful and happy that someone came and helped us.
"If Kian didn't catch his breath it could have been fatal."
After going home to her family - fiancé Ben Ireland, 26 and her five-year-old daughter Tia - Whiteside emailed the Lancaster Sainsbury's supermarket to complain about their lack of help and support.
Whiteside and her fiancé Ben Ireland with son Kian
Sainsbury's got back to the mother acknowledging her complaint, offering £100 in vouchers and stated that they are currently investigating the incident.
Whiteside said: "It was the scariest moment of my life. My baby could have died. I'm disgusted that employees just walked past looking and did nothing.
"I was in despair and needed help. I don't even have the stranger's name, but he is the reason my child is alive."
Sainsbury's said they are aware of the incident and are investigating.
Sainsbury's reply to Whiteside
A spokesperson said: "We have spoken to Natalie to apologise for the poor experience she received and we look forward to welcoming her back in our store.
"Our colleagues are always on hand if customers need help with their shop and we apologise for falling short of this on this occasion."
Whiteside added: "The whole experience has left me a bit shell shocked, but I'm just thankful my baby is still breathing."