A clean sheet from the Greysouthen star saw City claim the trophy, with Antoine Semenyo's second-half goal decisive.
It meant a second cup triumph for Trafford this season, after he also helped City win the Carabao Cup in March.
And his success with City meant Cumbrian goalkeepers have won the FA Cup in successive seasons, after Dean Henderson's success with Crystal Palace in the 2025 final.
Trafford in action during the Wembley final (Image: PA)
Trafford got the nod in goal from boss Pep Guardiola, continuing the 23-year-old's regular role in the XI in cup competitions this season.
And the former Carlisle United academy prospect helped keep Chelsea at bay as he recorded a second clean sheet in consecutive major finals.
Semenyo got the crucial goal with a brilliant flicked finish in the 71st minute.
Trafford was not called upon for many significant saves as City successfully kept Chelsea at bay.
The Cumbrian's landmark domestic double could now be followed by a major moment for Trafford on the international scene.
He is strongly tipped to be included in Thomas Tuchel's England squad for this summer's World Cup, when the squad is announced next Friday.
National team boss Tuchel was at Wembley to watch the FA Cup final. Whitehaven star Henderson is also expected to join Trafford in the Three Lions squad, along with regular No1 Jordan Pickford.