The 16-year-old singer, who was recently crowned the champion of this year's Australian version of The X Factor, was diagnosed with 'glue ear' syndrome when he was only seven years old.
The condition is reportedly caused by a chronic build-up of fluid in the middle ear and is the biggest cause of hearing loss in young children.
Mastin's mum told New Idea magazine: "It was an awful time. I remember he had this bad earache. Then his ear burst and white, gooey stuff went everywhere. It was horrible. [The doctor] did a hearing test and it was just a flat line. Nothing. Reece was deaf."
The teenage star had grommets put in his ears, but remained deaf until he underwent surgery a few months later that successfully restored his hearing. Mastin said: "Looking back, I've figured out how lucky I was."
He also said that now he has reached number one in the ARIAs singles chart and is recording his debut album, he is "doing something [he] always wanted to do".
> Simon Cowell 'approves' of 'X Factor' Australia
Listen to Reece Mastin's debut single 'Good Night' below: