The influence of nanoparticle agglomeration plays a crucial role in altering the mechanical properties of the resulting nanocomposites. The pre-processing of nanoparticles prior to its addition in the matrix alleviates the agglomeration and helps to attain the uniform dispersion. In the present work, an ultrasonic-assisted stir/squeeze casting technique was employed to prepare the aluminum 6061 (Al6061) based hybrid nanocomposites reinforced with pre-processed TiO2 and multi-walled carbon nanotubes (MWCNTs). In order to achieve uniform dispersion of nano-reinforcements on resulting Al-based hybrid nanocomposites, four different reinforcements pre-processing routes, i.e. sonication, ball-milling, stirring and its combinations were adopted. The effects of numerous parameters such as reinforcement weight percentage, reinforcement pre-processing routes and fabrication techniques on the microstructure and mechanical properties of the hybrid nanocomposites were investigated. The effect of various pre-processing routes on the morphology evolution in MWCNTs and TiO2/MWCNT nanoparticles mixture were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. Further, the microstructural investigation of the prepared nanocomposites was carried out by employing optical microscopy (OM) and x-ray mapping analysis. Mechanical characterizations such as tensile strength and microhardness show significant enhancement in the mechanical properties of hybrid nanocomposites due to the addition of pre-processed nano-reinforcements. The ultimate tensile strength (UTS), yield strength (YS) and microhardness of the ultrasonic-assisted squeeze cast Al6061-TiO2 (1 wt%)-MWCNT (1.5 wt%) hybrid nanocomposite remarkably enhanced by 302%, 400% and 77 %, respectively, due to MWCNTs shortening, uniform dispersion of reinforcements and processing technique. The fractured surface morphology of hybrid nanocomposites was examined using Field Emission-SEM, which exhibits the presence of nano-reinforcements without any sign of destruction in their morphology. © 2020 IOP Publishing Ltd.