From time immemorial humans have felt the need to explore themselves, their beliefs and all that surrounds them in nature in as diverse ways as possible. This has allowed artists to make use of varied mediums. This art was popularized in the 20th century when artists began to bend the rules of traditional high arts such as painting and sculpture.
However, mixed media concepts can be found throughout time as long as one considers any artwork using more than one type of material (or medium) to be classified as mixed media art. Today, mixed media art is popular among both professional artists as well as hobbyists who enjoy "crafting" projects such as collage and card making and scrapbooking. It is often referred to as assemblage art which refers to both professional and hobbyist activities related to mixed media concepts.
Many of the most popular pieces completed using mixed media art are collages, altered objects and books, 2D and 3D assemblage, artist trading cards (ATCs), greeting cards, journaling, and bookmaking.
A classic example of mixed media art would be a canvas painting done with ink, paint and collage.
Similarly mixed media in craft would be making a stationery box where the base is your recycled shoe box, you have painted it with colors or stuck newspaper to cover it and embellish it with sequins, stones or buttons. We see a coming together of varied materials to make up a stationery box.
Mixed media is an art and craft form that gives that platform for both professional artists and hobbyist alike to create all the same.