Ground tire rubber (GTR) from end-of-life tires was mechanically devulcanized in a lab-made two-roll machine. Parameters such as rolling speed and number of rolling cycles were systematically changed, and their effects on the degree of devulcanization and on the chemical structure of the resulting materials were investigated. Milder devulcanization conditions promoted a selective scission of S–S bonds inside the macromolecular structure of GTR, whereas harsher processing parameters led to a more pronounced and undesired C–C and C–S scission in the rubber main chains. Next, two different amounts of GTR and devulcanized GTR (dGTR) were added through melt compounding to an EPDM. The effect of the devulcanization parameters and of the GTR/dGTR loading on the microstructural and thermomechanical properties of the resulting compounds was systematically investigated. A better interface was found for dGTR than GTR, due to revulcanization in which the restored cross-link sites could form chemical bonds with the EPDM matrix; thus, energy absorption and strain at break increased. The possibility to produce novel environmentally friendly EPDM compounds with tailorable properties with respect to the neat matrix and with a lower cost and raw material amount has thus been demonstrated.