Liam Booth, 18, a member of St John Wales in Llandudno, stopped his car to offer help when he came across the accident on the A55 in nearby Colwyn Bay.
He performed CPR until paramedics arrived to take the man to hospital.
St John Wales is offering free life saving awareness sessions this month.
Recalling the incident, Liam said: "I knew what to do thanks to my training. For people who might not have the skills or the confidence to help, a quick first aid course could really be the difference."
St John Wales says only one in 10 people have the necessary skills to save a life.
Chief executive Keith Dunn said: "We believe that no one should die because they needed first aid and didn't get it. That's why our aim is to have a first aider in every home in Wales.
"Liam's training and presence of mind to act calmly and effectively in a difficult situation shows just how valuable first aid training can be."
A volunteer first aider who helped save a man's life following a road crash in Conwy county is urging others to learn basic life saving skills.