The two-time Pro Bowler was one of the Dolphins' final cuts Saturday as the team got down to the 53-man roster limit. The move may have been prompted by an injury to rookie running back Daniel Thomas, who sat out practice Thursday.
Johnson practiced Thursday and said he was glad to get another shot at a comeback.
"I could have easily said I'm going to retire as a Chief," he said. "You knew coming back it was going to be a roller-coaster ride, and you expect that. You roll with the punches.
"I love playing and it's always going to be in me. I haven't lost that fire yet to be under the lights and to compete to be the best."
Newcomer Reggie Bush will start at running back in Miami's season opener Monday against New England.
Johnson sat out most of last season after being released by the Washington Redskins in September. He has career rushing totals of 6,221 yards and 55 scores, but his most recent touchdown came in 2008.
He was drafted in the first round by Kansas City in 2003, and his time there was marred by off-field problems, including two suspensions in his final 12 months with the team. The Chiefs released him in 2009 after he posted a gay slur on his Twitter account and questioned the competence of coach Todd Haley.
When he signed with the team last month, Johnson cited his background as the reason he remained out of work this summer until almost four weeks into training camp.
"I don't think it had anything to do with talent," he said. "I understand you want your best guys to represent the organization. You go with the guy with the cleaner slate. That's what I realized going through this process. All you can do is be thankful for one little shot."
Johnson's best years were 2005-06 with the Chiefs, when he had consecutive 1,750-yard seasons and ran for a total of 37 touchdowns.
After they released him in November 2009, Johnson played in seven games as a reserve in Cincinnati. He signed a three-year, incentive-laden contract with Washington in March 2010, but gained only two yards on five carries in two games before being cut by the Redskins.
Information from The Associated Press was used in this report.