Protein homeostasis collapse typically leads to protein aggregation into amyloid fibrils and diffuse amorphous aggregates, which both occur in Alzheimer’s and other neurodegenerative diseases, but their relationship remains to be clarified. Here we examine the interactions between the amorphously aggregated non-chaperone proteins (albumin, β-lactoglobulin, and superoxide dismutase 1) and Alzheimer’s amyloid-β (Aβ) peptides. Amorphous aggregates suppress the primary nucleation and elongation of Aβ fibrillation and modulate Aβ toxicity. The higher inhibitory efficiency of intermediately sized molten globular aggregates (20–300 nm) on Aβ fibrillation is hypothesized to be due to the higher amount of exposed hydrophobic residues and higher free energy. The formed co-aggregates are off-pathway species that favor formation of the amorphous end state instead of fibrillar amyloid structures normally formed by Aβ. Our findings expand our knowledge of how the native and aggregated cellular proteins modulate Aβ aggregation at the molecular and mesoscopic level and point out the major conclusions.