Ian "Jacko" Roberts, 54, of Acrefair, near Wrexham, was in Toulouse for the historic 3-0 win against Russia.
But on Monday, he will return to hospital for treatment on a tumour in his nasal passage.
He said of the game: "The atmosphere was amazing...exceptional."
The win means Wales qualified for the second round, with their next game to be played in Paris on Saturday.
And Mr Roberts, a father-of-one, said he got to meet former Wales international players turned pundits Robbie Savage and John Hartson, who has had cancer, at his hotel in Toulouse.
"I was having a bit of a talk to John Hartson and he was giving me the arm around the shoulder. It was really good," he added.
Two weeks ago Mr Roberts finished 12 sessions of chemotherapy and is about to begin radiation treatment.
The carpenter and joiner, who sings for Froncysyllte Male Voice Choir, was captured singing the national anthem by Hartson who tweeted the footage before the Russia game.
He said it had been brilliant watching the team in France, adding: "Especially after all the years where we have had the heartbreaks, all them times when we failed to qualify.
"It was actually the morning of the Andorra game [a Euro 2016 qualifier which Wales won 2-0 in October] I was told I had the tumour in the nasal passage.
"For all the winter I didn't know I was coming [to France] until the Wednesday before.
"And my friends have all got me tickets. They've all chipped in money for me to come because obviously I've been out of work for a while."
Mr Roberts said it has been hard travelling but he has had a great time with his friends, including Gareth Roberts, 55, originally of Bangor but now living in Chester.
Mr Roberts said he has met lots of footballers and stars through his singing but it is nice for his son, Thomas, 14, to see him mingling with such big names.
A football fan who has taken a break from his cancer treatment to follow Wales in France in Euro 2016 said it has been "amazing".