Three dimensionally shaped leather is a very ancient art. Leather drinking vessels, bottles, boxes, and other objects were fairly common in the middle ages, and heavier leather shaped into armor was used in ancient Greece and Rome. To the best of my knowledge, in recorded history, leather was first used extensively for mask making in the early 16th century, when such masks were created for the performers of the Commedia dell'Arte.
After the 17th century, when paper mache became a popular mask medium, the art of leather maskmaking began to disappear, preserved only by a few isolated artisans in Europe.
In the late 1970s, the rising popularity of Renaissance festivals and re-enactment groups like the SCA brought with it a resurgence of interest ancient crafts, and many craftspeople began researching the techniques of their historical antecedents. Today, there are artisans and craftspeople the world over who produce many beautiful objects using technologies that date to the Renaissance, the middle ages, or even earlier. Among them, there are now quite a few who sculpt leather masks.
The Commedia masks were shaped of fairly heavy leather, dampened and stretched over rigid wooden forms, and used only a half-dozen or so basic designs which represented the standardized characters of the commedia plays. Maskmakers of today, of course, are not so constrained, and the variety of their designs is limited only by their own imaginations.
Modern leather maskmakers may add their own refinements and variations to the ancient art, and use dyes and paints of modern manufacture, but the basic technique remains essentially the same as it was in the Renaissance. Vegetable tanned leather is wetted (making it pliable and stretchable), and shaped over a wooden or plaster base, sometimes using tools to achieve particular shapes, or carve designs into the surface of the leather.
In my own work, plaster and wooden molds generally serve only as a starting point, to ensure that the masks have the right basic form to fit comfortably on a human face. Most of the actual shaping and sculpting of the leather is done by hand. Once the mask is shaped, a stiffening agent is applied, and the mask is sealed, and primed.
Since moisture is what allows the leather to be shaped, the finished mask must be protected from re-wetting, lest it lose it's shape. Historically this was done with pitch, wax, or a combination of wax and oil. I use an acrylic sealer, and then paint the mask using acrylic paints, adding a final coat of acrylic varnish to protect the painted surface.