The Cowboys signed Johnson on Monday, Aaron Wilson of KPRC 2 Houston reports.
Per Patrik Walker of the Cowboys' official site, Johnson was one of several players to go through a workout Monday and presumably impressed enough to earn a contract ahead of mandatory minicamp. The undrafted wideout out of Oklahoma State caught 20 of 26 targets for 398 yards and three scores across 12 appearances as a rookie with the Chargers, but he's played in just 14 games with three different teams (Jaguars, Raiders and Texans) over the past two years. In a corresponding move, Dallas cut Antonio Callaway.