Johnson's most recent trouble occurred at 3:30 a.m. Friday in Gilbert, Ariz., where police say they detained Johnson for speeding and suspicion of driving under the influence.
Johnson had been suspended by the NFL for the first eight games of the 2007 season for violating probation on a gun charge. He spent two months in jail and was released in May.
"We are upset and embarrassed by Tank's actions last week," general manager Jerry Angelo said in a news release. "He compromised the credibility of our organization."
Johnson's departure coincided with the NFL's rookie symposium in Florida. Commissioner Roger Goodell briefed all 255 members of this year's draft class yesterday on the league's stricter conduct rules.
"We're concerned about them as men," Goodell said at the symposium shortly before Johnson's release. "How do they become not only great NFL players, but how do they become great men? How do they conduct themselves appropriately for the remainder of their life, not just when they're in the National Football League?"
The rookie symposium teaches incoming NFL players - attendance is mandatory for draft picks - about how to handle finances and relationships and how to prepare for life after football. But the conduct issue is one of Goodell's priorities, and it's taking center stage this year.
He released his stricter policy in April, an attempt to quell a series of off-the-field episodes involving NFL players, notably cases involving three suspended players - Tennessee Titans cornerback Adam "Pacman" Jones, Cincinnati Bengals receiver Chris Henry and Johnson.
Before learning more details of Johnson's traffic stop, Bears coach Lovie Smith brought the player into his office yesterday at Halas Hall in Lake Forest and told him the Bears' baby-sitting job was over.
Johnson had flown to Chicago from his home in Arizona, already too late to talk his way out of the relatively minor indiscretion.
Smith was among the 147 names on Johnson's county jail visitor list and was a strong supporter, frequently referring to Johnson as "a good person." He was particularly disappointed by the latest development.
"A lot of people within our organization gave extra time and energy to support Tank," Smith said in the release. " ... Ultimately, Tank needed to live up to his side of the deal."
By itself, driving 40 mph in a 25-mph zone at 3:30 a.m. and "being impaired to the slightest degree," in the words of Gilbert police spokesman Sgt. Andrew Duncan, would not be enough to terminate employment immediately. But the Bears have been holding Johnson's hand since June 11, 2005, when he was charged with unlawful possession of a handgun after Chicago police found a loaded 9-mm Ruger in his parked car outside a downtown nightclub.
Cornerback Nathan Vasher, one of several Bears players who visited Johnson in jail, understood why the team's patience toward his troubled teammate finally ran out.
"The Bears were kind of pushed into a corner. ... I think you have to be accountable for your actions, and the Chicago Bears had to do what was needed.
"I am surprised. I was really convinced that Tank had more than thought about the time he did in prison or just every other compromising situation he has been in, not to jeopardize [his career] any further."
Don Pierson writes for the Chicago Tribune. The Associated Press contributed to this article.