Thomas, 64, announced on Twitter that he was beginning treatment on Monday after receiving the diagnosis in January.
“Hi everyone, today is the day I start my treatment and my fight back against the big C,” Thomas posted on his personal Twitter account.
“I aim to beat this in 90 minutes with no extra time and penalties.
“I have been blown away by the love, kindness and support I want to thank you all from the bottom of my heart.”
A crowdfunding page has been set up to help Thomas, with over £13,000 raised within three days of being established.
Thomas won 51 caps for Wales and spent three years at United between 1978 and 1981, making over 100 appearances for the Red Devils.
The winger also played for Everton, Brighton, Stoke, Chelsea, West Brom, Derby, Shrewsbury and Leeds, as well as spending nine seasons with Wrexham across two spells.
Thomas famously scored the equaliser with a superb free-kick when Wrexham, then of the fourth tier, knocked out league champions Arsenal in the 1992 FA Cup third round.
He made his final appearance as a 38-year-old in 1993.
Thomas has worked in the media since retiring and is still a regular presence at United games.