Counsell, a 44-year-old Milwaukee-area native, spent the final five seasons of his 16-year big league career with the Brewers, retiring after the 2011 season. He has no previous managing or coaching experience.
"It's an honor, and it's humbling, but I feel like this is what I was meant to do," Counsell said. "I think I'll be better at this than I was at playing."
Counsell was given a contract through the 2017 season.
"He played the game with a chip on his shoulder and he played the game to win," Brewers general manager Doug Melvin said during a news conference. "He has a real edge for preparation."
A two-time World Series champion, Counsell scored the winning run for Florida in the 11th inning of Game 7 of the 1997 World Series and was MVP of the 2001 NLCS for Arizona.
A major league-worst 7-18, the Brewers lost 40 of their final 56 games under Roenicke. The skid included a late-season collapse last year, after they led the NL Central for nearly five months, and a 2-13 start this season.
"You think you could win two games in a row by mistake, where the other team's playing bad," Melvin said. "That's not acceptable, and it's hard to understand why."
Counsell became a special assistant to Melvin in 2012 and also was a part-time broadcaster for Milwaukee last season. Counsell was among the candidates last offseason to succeed Joe Maddon as Tampa Bay's manager.
"Are we a contending team right now? We're not," Counsell said. "We can't start over. Our record is our record. ... We can start being the team that we want to be."
Counsell had a .255 average in the big leagues. He knows something about struggling, going hitless in 45 consecutive at-bats in 2011.
The new manager's father, John Counsell, was a former minor league outfielder who worked for the Brewers from 1979 through 1987, running the speakers bureau and then becoming community relations director.
Milwaukee started 20-7 in 2014 and spent 150 of the regular season's 183 days alone in first or tied for the NL Central lead, including every day from April 5 through Aug. 31. The Brewers skidded to a 9-22 finish and wound up third in the division, eight games back of St. Louis and two behind Pittsburgh.
Roenicke became the first manager fired 25 games or fewer into a season since 2002, according to STATS. Detroit's Phil Garner (six games), Milwaukee's Davey Lopes (15), Colorado's Buddy Bell (22) and Kansas City's Tony Muser (23) were all let go quickly that year.
On March 19, Milwaukee exercised its 2016 option on Roenicke. But Melvin met with team owner Mark Attanasio on the off day last Thursday and discussed a possible change.
"It didn't feel good," Melvin said. "Slept on it for a day or so, and then just decided to make the change."
In 2011, in his first season as a major league manager, Roenicke led the Brewers to a 96-66 record -- the best in team history -- and the NL Central title. The Brewers beat Arizona in the first round and lost to St. Louis in the league championship series.
Milwaukee was 83-79 in 2012, 74-88 in 2013 and 82-80 last season.
"We have good players," Counsell said. "I hear it from other teams that we have good players."