From wiki:
The wall was part of the original ballpark construction of 1912. It is made of wood, and was covered in tin and concrete in 1934. It was then covered with hard plastic in 1976. A manual scoreboard is set into the wall. Despite the name, the Green Monster was not painted green until 1947; before that it was covered with advertisements. The Monster designation is relatively new. For most of its history it was simply called "The Wall". The wall is the highest among current Major League Baseball fields, and is the second highest among all professional baseball fields (including minor leagues), falling approximately six inches short of the left field wall at the Santander Stadium in York, Pennsylvania.
Youtube:
https://www.youtube.com/watch?v=_lq-xy7ifgA