Simmons worked for a mortgage company and spent some time as a probation officer. In 2010, he accepted an offer from Rex Ryan as a fellowship coach with the New York Jets.
Simmons was the assistant defensive line coach for the St. Louis/Los Angeles Rams. The Rams had 217 total sacks during Simmons' tenure. With the Rams, Simmons coached three-time Pro Bowl defensive tackle and the 2014 NFL Defensive Rookie of the YearAaron Donald, and defensive end Robert Quinn, who set a single-season franchise record with 19 sacks in 2013. In January 2017, he was hired by the Cleveland Browns to coach their defensive line.