Alexandra Lucy Hoegh's mum Dana, 37, told an inquest she was in the kitchen with a friend when "I heard a scream and then another scream. We both rushed to the door of the kitchen and met (Alexandra's nanny) Melinda there with Alexandra in her arms.
"I ran downstairs into the street. I asked my friend to call an ambulance and started mouth-to-mouth on her on the pavement.
"Her colour came back straight away but she did not respond in any way that gave me cause to think she was breathing."
She said paramedics arrived around 15 minutes later but the girl died later in hospital. The inquest heard that nanny, Melinda de la Cruz, had found Alexandra suspended and unconscious in her bedroom at the family's £12 million west London home on October 29 last year.
Dana – who is married to shipping tycoon Morten Hoegh, one of Britain's richest men - told Westminster coroner's court she had been about to leave for a medical appointment when she heard a scream.
Detective Sergeant Kevin Harwin said two officers arrived to distressing scenes.
He told the court: "The child concerned was already in the rear of the ambulance being treated by four paramedics. Also at the scene was Alexandra's mother and a friend, and a nanny Melinda - one of two nannies looking after the child.
"Two females were crying hysterically and the friend was collapsing and retching. All parties were in a serious state of shock."
The inquest heard that Alexandra had been at playgroup, fed and put to bed by Andrea de la Cruz, a cleaner and nanny, before her daughter Melinda arrived to take over at around 1.45pm. She said she had been 'very happy', and could hear on the baby monitor that she was reading a book to herself.
The court heard she would use a small table near the crib to climb in and out, and although Alexandra was tall enough to stand up and reach the roller blind, she had never done so before.
Melinda said the monitor was 'very quiet' when she arrived and after speaking briefly to Alexandra's mother, she went upstairs to wake the child.
She told the court that she found Alexandra standing in her crib with the beaded cord around her neck. She said she could not remember if she was breathing.
"I am panicking. I picked up Alexandra and called downstairs and said something is happening," she said.
US-born Dana was taken to hospital to be treated for shock. Her husband, 39, who is the chairman of the multi-billion-pound oil and gas shipping firm Hoegh LNG based in Norway and has a personal fortune estimated at £175 million.
Recording a verdict of accidental death, coroner Fiona Wilcox said: "This is clearly an absolutely tragic and appalling death and I can't imagine how the family must feel."
She added that she would carry out an inquiry with the Health and Safety Executive to find out whether such roller blinds have warnings with them about the risks to children from the cords.
Please, if you have a blind cord anywhere near your baby's cot or toddler's bed, get rid of it now.