Mikell's contract was due to count $9 million against the cap -- more than double what it was in each of the previous two seasons.
The Rams must account for $6 million in prorated bonus charges over the next two seasons whether or not Mikell is on the roster.
Mikell forced four fumbles last year and had career highs of 101 combined tackles and three sacks in his second season in St. Louis.
Prior to joining the Rams, Mikell played eight seasons for the Philadelphia Eagles. He has played in at least 14 games in each year.
Mikell joins a lengthy list of unrestricted free-agent safeties that will include Ed Reed of the Baltimore Ravens, Dashon Goldson of the San Francisco 49ers and LaRon Landry of the New York Jets when free agency officially opens on Tuesday at 4 p.m. ET.
The Rams also parted ways with safety Craig Dahl, an unrestricted free agent.
ESPN NFL Insider Adam Schefter, ESPN.com's Mike Sando and The Associated Press contributed to this report.