Wearing a red baseball cap and white T-shirt, Johnson walked out of Cook County Jail smiling broadly. He did not speak to reporters before getting into a sports utility vehicle and leaving.
He was released on good behavior after serving half his four-month sentence. The 25-year-old player is to have a hearing with NFL commissioner Roger Goodell on Wednesday in New York and could face a multigame suspension by the league.
Johnson's jailing in March stemmed from a Dec. 14 police raid on his Gurnee home, where authorities found six unregistered firearms.
Johnson pleaded guilty last month to a misdemeanor weapons charge as part of a deal with prosecutors that kept him from serving more time in jail. He was ordered to serve 45 days, which he was able to serve concurrently with the sentence for violating his probation.
At the time of the December raid, Johnson was on probation in a November 2005 Cook County case. In that case, he pleaded guilty to a misdemeanor charge stemming from an arrest in which a nightclub valet reported seeing Johnson with a handgun in his SUV.
Johnson's bodyguard, Willie B. Posey, was arrested on felony drug charges after the raid on Johnson's home. Posey was shot and killed two days later during an early morning fight while he and Johnson were at a nightclub in Chicago.
The Bears suspended Johnson for one game for being at the club.