Poly (methyl methacrylate) (PMMA) and acrylonitrilebutadiene-styrene (ABS) - multiwall carbon nanotube (MWNT) and chopped carbon fiber (CCF) composites were prepared by a melt mixing protocol at various concentrations. Specimens were fabricated and tested using constant amplitude-of-deflection fatigue testing. The numbers of cycles to failure were recorded and analyzed using the linear version of the 2-parameter Weibull model. In the PMMA matrix, the 1.0vol% MWNT reinforced composites outperformed the neat PMMA matrix by +396% while the 1.0vol% CCF composites increased fatigue life by +198% over the control. The increase in fatigue life may be attributed to the nanoscale dimensions of the MWNTs. This enables them to directly interact with the matrix at the sub-micron scale where damage such as crazing begins, which ultimately initiates a critical crack that leads to failure of the specimen. The ABS composite specimens did not show any increase in fatigue life. The underlying reasons for the lack of fatigue improvement remain unclear.