The aim of this study was to evaluate the mechanical properties (flexural strength (FS), flexural modulus (FM), Vickers microhardness (VMN), and shear bond strength (SBS)) of preheated composites. Two preheated composites (Z350XT and Proclinic) and one self-adhesive resin cement (RelyX™ U200) were used to fabricate specimens. All the specimens were subjected to thermocycling before their mechanical properties were evaluated. One-way ANOVA was used for statistical analysis, followed by Tukey’s post hoc test. The chi-square test was used to evaluate the failure modes after SBS test. Results: RelyX™ U200 had a significantly higher FS (106.22 ± 14.23 MPa) than Proclinic (85.76 ± 12.75 MPa) and Z350 (71.47 ± 22.98 MPa). Z350 (118.10 ± 11.3 GPa) and RelyX™ U200 (110.88 ± 13.44 GPa) had significantly higher FMs than Proclinic (83.72 ± 9.3 GPa). A significantly higher VHN was seen with Z350 (136.84 ± 11.52 VHN) compared to Proclinic (115.25 ± 17.15 VHN) and RelyX™ U200 (100.83 ± 12.69 VHN). Z350 had a higher SBS (20.75 ± 5.6 MPa) than RelyX™ U200 (15.4 ± 3.46 MPa), while Proclinic was the weakest among all the groups (6.76 ± 1.44 MPa). In the failure mode analysis, the mixed failure mode was predominantly seen in all groups. In conclusion, not all preheated composites behave the same and it is the clinician’s responsibility to select the appropriate material for every clinical situation.