Some new types of Ce3+ and Dy3+ co-doped manganese-zinc nanospinel ferrites (CDMZNSFs) of the form (Mn0.5Zn0.5)[Fe2 − 2xCexDyx]O4 (with 0.0 ≤ x ≤ 0.1) were sonochemically produced and characterized. The structure, morphology, optical and magnetic behaviors of these NSFs were determined as a function of co-dopant (Ce3+ and Dy3+) contents. In addition, the bactericidal (on the gram-positive and gram-negative bacterial strains) and anti-cancerous effectiveness of these NSFs were assessed. The cancer cells' growth inhibitory action of these NSFs was tested against both normal (HEK-293) and cancerous (HCT-116) human cells. The direct optical band gap energies of the studied NSFs were ranged from 1.54 to 1.85 eV. The measured hysteresis loop and temperature-dependent magnetization of the NSFs disclosed a phase transformation from the superparamagnetic (SPM) at room temperature (RT) to ferromagnetic (FM) below the blocking temperature (TB). The saturation magnetization and TB were decreased with the increase in co-dopant contents. After 48 h of treatment of the cancerous cells with the NSFs, their population was significantly dropped as shown by the MTT assay, indicating the selective inhibition of the cancer cells growth by the proposed NSFs. Conversely, the non-cancerous cells (HEK-293) population remained unaffected. The IC50 values of the NSFs-treated cancerous cells (HCT-116) were in the range of 0.74–2.35 µg/mL. The results of the MIC and MBC assays revealed the reasonable antibacterial efficacy (growth inhibitory activity) of these NSFs when tested against the E. coli and S. aureus bacterial strains. It is established that the proposed Ce3+/Dy3+ co-activated CDMZNSFs may be beneficial for the anti-cancerous and bactericidal applications.