Mason celebrated his full debut by setting up the opener for Christian Kabasele as the Hornets eased into the fourth round of the FA Cup with a 2-0 victory at Vicarage Road.
The 19-year-old full-back weaved into space down the left wing before producing a dangerous cross that was finished by Kabasele.
"Mason played wonderfully on his full debut with an assist for the goal," Mazzarri said.
"We had Mason with us when we did pre-season. With young players it's very important for them to be with the first team and to make their debuts because this will make them grow as players.
"Sometimes it's fundamental to see what we've got at home in our own house and make these young players grow so that we don't always have to go abroad and sign players for a crazy amount of money."