Deer was drafted by the San Francisco Giants in the 4th round of the 1978 amateur draft. Deer made his debut on September 4, 1984 at Candlestick Park against the Cincinnati Reds. As a pinch hitter facing Ted Power in the ninth inning, he flied out to end the game. He played 13 games that year, batting.167 while having three home runs and RBIs, with seven walks and 10 strikeouts. The following year, he played in 78 games, batting.185 while having eight home runs, 20 RBIs, 71 strikeouts, and 23 walks. On December 18, he was traded to the Milwaukee Brewers for two minor league prospects. With Milwaukee, he had increased playing time. In 1986, he played in 134 games, having a.232 batting average while having 33 home runs and 86 RBIs, although he had 179 strikeouts and 72 walks. The following year, he played in 134 games while batting.238 with 28 home runs and 80 RBIs. He had 12 stolen bases, 186 strikeouts, and 86 walks, the former two being career highs. Notably, he hit a game-tying home run on Easter Sunday, to give the Milwaukee Brewers their 12th straight win to start the season. The home run was featured on the cover of Sports Illustrated. He shares the dubious honor of having the second-lowest official batting average while still qualifying for the batting title when he batted.179 in 1991. He is the only player since 1901 to have a batting average less than.215 in at least 400 at-bats in at least four seasons. Deer held the American League record for strikeouts in a season until being passed by Jack Cust in 2008, and had at least 140 strikeouts on seven occasions. Deer averaged a strikeout every 2.75 at-bats. Deer has also gained some notoriety among studiers of baseball statistics due to his propensity for the Three True Outcomes. Because of his ability to hit home runs and take walks and better-than-average fielding ability, he remained a valuable player despite his inability to hit for average, as evidenced by his career 13.7 Wins Above Replacement. His final major league appearance was in the bottom of the eighth inning of a Padres-Cardinals game on August 5, 1996, replacing Greg Vaughn in left field. He caught a flyball to end the inning for the Padres, who lost 8-2. Since his playing days, Deer has had a career in drag racing, sprint car racing and has served as a roving hitting instructor for the San Diego Padres minor league system and is currently the owner of Viz-U-Bat. On November 26, 2012, Deer was named the assistant hitting coach for the Chicago Cubs, under his friend Dale Sveum, manager of the Cubs. They were Milwaukee Brewers teammates from 1986–1990. He is also very close friends with former Brewers teammates Chris Bosio and Robin Yount. While with the Brewers, he worked for his father's construction firm.
