Alexander was signed as an undrafted free agent out of Berkeley St. Mary's in 2005 by the Carolina Panthers. He spent that season on the Panthers' practice squad and was released in September of 2006. He then signed, for five days, with the Baltimore Ravens before eventually being signed to the Washington Redskins practice squad.
He made his NFL debut in Week 5 of the 2007 season for the Redskins and made his first career start that same season in Week 10 against the Eagles. He spent the majority of the 2007-2009 seasons on special teams. It wasn't until 2010 that Alexander began to rotate in on a regular basis at outside linebacker for the Redskins and he played in all 16 games over the last two seasons. Lorenzo's performance in 2012 was good enough to get him sent to the Pro Bowl for the first time in his career.
In his time in the league, Alexander has made 162 tackles, eight sacks and has broken up five passes. It's currently unclear whether or not the Cardinals plan to expand Alexander's role on the team's defense or use him primarily as a special teams player as the Redskins did.