Actors wore masks like this during the Mystery Plays. These plays were Bible stories performed by guild members to teach people about the life of Christ. They were held once a year on a public holiday.
This leather mask has eyebrows scored on it and the smiling mouth would have curved around the wearer's face. It may have been brightly painted or decorated. Unfortunately, we don't know what character this mask represents.
You can see this mask on display in the medieval section of the History Galleries. You can also listen to a Mystery Play and try on your own mask!