According to the Red Sox promotions department, Wally was a huge Red Sox fan who, in 1947, decided to move inside the left field wall of Fenway Park, since it "eats up" hits that would easily be home runs at other parks. Apparently, he was very shy and lived the life of a hermit for 50 years. In 1997, on the 50th anniversary of the Green Monster being painted green, he came out of the manual scoreboard and has been interacting with players and fans ever since.
When the team began to grow out their beards as a trademark during their 2013 World Series run, Wally was given a long beard as well.
In January 2016, the Red Sox unveiled a new mascot named Tessie, Wally the Green Monster’s little sister.