The Leinster fly-half has been named in the starting lineup for the first Test, which takes place in Brisbane tomorrow morning.
The 27-year-old told The Mirror: "Watching Jonny had a big effect on me. I realised that if I wanted to get to the top I needed to follow his example. I wanted to play rugby professionally but had no idea what that entailed. I didn't really do weights, I didn't really train that hard.
"I loved going out training with the ball but I never worked in the gym. Then I read stories about Jonny training even on Christmas Day and it inspired me to change my attitude and overhaul the way I trained.
"It's not easy when you are a young lad thrown into the number 10 jersey for your country. I looked at how he coped with it, how normal, how humble he remained, and that helped me. He is an unbelievably good role model."
Sexton will be one of eight players to face Australia with the Lions for the first time.