In 1970, the Brewers were still a new team and were having difficulty drawing spectators to their games at Milwaukee County Stadium. In late June, a 69-year-old fan named Milt Mason decided to sit on top of the scoreboard until the team could draw a home crowd of 40,000. He was there for about 40 days. When 44,387 people finally heeded his call and attended a game on August 3, Mason was welcomed back down and he became the original Bernie Brewer.
The Bernie Brewer character became the team's mascot in 1973, appearing as a cheerful man with a big moustache. A beer-barreled chalet was built for him inside the stadium where he led the crowd cheering. Following each home run and every victory by the Brewers, he would slide down and plunge himself into a huge beer mug in celebration. He was joined by a companion named Bonnie Brewer, who would playfully swat at the backside of the opposing team's third base coach with a broom as the field crew swept the basepaths.
Bernie Brewer was a fixture at Brewers home games until 1984, when the Brewers re-built the bleachers, replacing the chalet with a sound tower and sending Bernie into retirement. By popular demand, Bernie Brewer came out of his retirement in 1993, when the fans voted for his return. Bernie was brought back not as just a mustachioed man in lederhosen, but a full-body costume of a man, including large foam head. The chalet was then rebuilt (it had been in storage) above the left-center field bleachers.
At Miller Park, the current home of the Milwaukee Brewers, Bernie Brewer has his own "dugout" atop the bleachers behind left field, where he continues to lead the cheering at every home game. Currently he slides down a plastic yellow slide when a Brewer hits a homerun, while a sign tower with Bob Uecker's trademark homerun call ("Get up, get up, get outta here, GONE!!") lights up above the Dugout.
Much of the information in this article is taken from Bernie Brewer's Wikipedia page