The ad, created by Bartle Bogle Hegarty, ran in September and centred on the fact that up to 140,000 people die each year in situations where first aid could have saved their lives.
It features a father going through the process of being diagnosed with, being treated for, and surviving, cancer.
The character dies at the end of the ad by choking on a piece of food at a barbecue, as his daughter looks on.
A total of 144 people complained, with complaints focusing on the ad causing distress, as well as offending people with cancer, and their families.
However, the ASA said the ad was justified, given it had a post-9pm timing restriction.
The ad ended with the strapline: "First aid could help prevent up to 140,000 deaths every year. The same number that die from cancer. Be the difference. Text 'HELP' to '84025' for a free first aid guide."
Media planning and buying was handled by John Ayling & Associates.
Scott Jacobson, director of marketing, communications and fundraising at St John Ambulance, said: "We wanted this campaign to really stun people with how needless it is for anyone to die in a situation where the most basic first aid might have saved them.
"People know that cancer is a very serious illness and take steps to prevent it and look after loved ones who are stricken with it, so we hope that the tragedy of someone surviving cancer, but then dying because he needed first aid and didn’t get it, will raise awareness that a lack of first aid is just as life-threatening and will motivate people to text in for a free first aid guide."Follow @DanFareyJones