Allen wrote about the move on Twitter: "Thank you to all the phin fans you've been great!!! It's been a gr8 ride.. Much love!!"
Johnson signed with the Dolphins on Aug. 23 after sitting out most of the 2010 season following his release from the Washington Redskins. The 2003 first-round draft pick of the Kansas City Chiefs has 6,221 rushing yards and 55 rushing touchdowns, but he has dealt with off-the-field problems throughout his career.
Allen, who started 53 games since joining the Dolphins in 2006, missed most of the last two seasons with knee injuries. He returned from a 22-month hiatus to play against the Tampa Bay Buccaneers in the Dolphins' third preseason game, but according to the Miami Herald, the Dolphins will save $1.5 million by releasing Allen.
The Dolphins also terminated the contract of fullback Lousaka Polite, an eigh-year veteran who started 24 games over the past three seasons.
It's possible one or more of the departing veterans will be re-signed after the first week of the season.
The Dolphins placed backup offensive tackle Lydon Murtha (toe) on season-ending injured reserve and waived two of their four quarterbacks: Pat Devlin and Kevin O'Connell.
The Associated Press contributed to this report.