Monica Stoffers, had taken her two-week-old daughter Ella into the feeding room in the branch in Plymouth’s Drake Circus shopping centre, when suddenly the lights went out.
"I managed to feel my way around to get my baby back in to her pram so I could find my way out of the room, once out (and very upset and distressed) I realised that every one had left and the store was completely shut," the mum-of-two wrote in a Facebook post about the incident.
Stoffers and her older child in 2014
Stoffers tried to attract the attention of a shopping centre security guard by banging on the window, but couldn't make enough noise for him to hear. So she made her way around to the shop door.
"I carried on banging with the alarms now going off too," she wrote.
"My baby was hysterical at this point and so was I almost."
It was then Stoffers finally managed to draw attention to herself and the security guard contacted a shop workers who let the distressed mother and daughter out.
Stoffers said the shop worker offered her "a very limp" apology.
Stoffers was unsure if the incident would have been reported to Mothercare's head office so she shared her story on the shop's Facebook page.
"I had a panic attack in the shop and was scared for my two-week-old baby," she wrote.
"I cannot believe this has happened and the staff that saw me enter the room did not come back to tell me they needed to shut the shop."
Following the incident Stoffers said Ella was "very stressed and we have had our first very bad night."
Mothercare responded in a statement: "Our store team provided comfort and support to a customer after they discovered she was inside the store as it was being closed. We appreciate that the whole experience must have been very upsetting for her.
"We will take the opportunity to ensure all our store teams have fully checked there are no customers in store at the time of closing, to avoid this happening again."