The two sides reached an agreement late Thursday, and completed the signing on Friday. The contract will span six-years, although additional details are not known.
Johnson participated in Detroit's morning session.
Johnson and his agent, Bus Cook, have been in Detroit since early this week in attempts to hammer out a contract.
One hangup in the negotiations is believed to be guaranteed money. Johnson's camp wanted $30 million in guarantees, but the Lions were not willing to go that high.
The former Georgia Tech standout was first team All-ACC each of his three seasons with the school, in which he accrued 178 catches, 2,927 yards and 28 touchdowns in 38 career games.
The trio has been catching rave reviews for its potential as the 2007 season approaches.