The Broncos had been sitting at 52 men on their active roster since quarterback Kyle Orton was waived by the team last week. Mohamed was activated after cornerback Cassius Vaughn was placed on injured reserve with a broken fibula.
The moves will allow Denver to sign two more members to their practice squad.
McCarthy was a preseason standout in 2011 and played in eight games for Denver in 2010 accounting for four tackles. McCarthy played college ball for Notre Dame and was signed as an undrafted free agent in 2010.
Mohamed was drafted by the Broncos in the sixth round of the 2011 draft out of California.
Both players will likely contribute very well to special teams.