Knight Frank office manager Lauren Williamson was taking a shower when she heard the frantic cries of her mother and grandmother.
The 26-year-old grabbed a dressing gown and rushed downstairs to find grandfather Roland slumped on the sofa, unconscious.
While her mother called for an ambulance, Lauren's first aid training kicked in.
She said: "He’d vomited. It was obvious he wasn’t breathing but I checked anyway before we dragged him onto the floor so I could start CPR.
"He vomited again so I put him in the recovery position but then he stopped breathing again so I started CPR again.
"I did about six rounds of chest compressions and rescue breaths before the paramedics arrived.
"The adrenaline kicked in and I just did what I’d been taught to do. The paramedics said it was a miracle that he survived."
Roland regained consciousness after paramedics used a defibrillator to deliver electric shocks to his heart.
He was in hospital for three weeks and underwent an operation to have a device that uses electric shocks to control irregular heartbeats fitted.
Following the heart attack, which happened in February, Roland said: "It’s lucky for me someone in the family had learned how to do CPR. There’s no doubt Lauren saved my life."
He was given the all clear by doctors earlier this month and is now busy working his way back to his former fitness regime involving a two-mile walk each day.
Canary Wharf worker Lauren, who lives with her parents and grandparents in Leigh-on-Sea had completed a three-day first aid course with St John Ambulance last year and did a refresher course last month.
Her trainer Jon Woods says: "We taught Lauren how to deal with life or death situations in the workplace.
"But her story just goes to show everyone needs to learn basic first aid because a medical emergency can happen to anyone at any time, anywhere."