Winners of 11 of their first 15 games, the Nets went into a tailspin in December, winning only three times in 13 games.
"We just didn't have the same fire that we had when we were 11-4," General Manager Billy King said.
Top assistant P.J. Carlesimo will serve as interim head coach beginning with Friday's home game against Charlotte.
Much had been expected of the team, which had moved from New Jersey to a sparkling new arena in Brooklyn and spent millions of Russian owner Mikhail Prokhorov's money in hopes of rivalling the New York Knicks.
But with point guard Deron Williams complaining about the offence and the losses mounting, the end for Johnson came a day after the Nets were routed 108-93 by Milwaukee.
"For some reason, he just wasn't reaching them (the players) anymore," King said.
Johnson had been the Nets' coach since June 2010. In two-plus seasons, he compiled a record of 60-116, going 14-14 this season as the Nets fell 6-1/2 games behind the Knicks in the Atlantic Division.
"This is a really disappointing day for me and my family," Johnson, 47, said. "This is a difficult time. It's something that I didn't necessarily see coming, especially after our pretty good November."
Before joining the Nets, Johnson had coached the Dallas Mavericks for four seasons, leading the 2005-6 team to the Western Conference championship.
He compiled a 254-186 record in seven seasons as an NBA coach.