The 42-amino-acid -amyloid (A42) is a critical causative agent in the pathology of Alzheimer’s disease. The hereditary Arctic mutation of A42 (E22G) leads to increased intracellular accumulation of -amyloid in early-onset Alzheimer’s disease. However, it remains largely unknown how the Arctic mutant variant leads to aggressive protein aggregation and increased intracellular toxicity. Here, we constructed stable cell lines expressing fluorescent-tagged wildtype (WT) and E22G A42 to study the aggregation kinetics of the Arctic A42 mutant peptide and its heterogeneous structural forms. Arctic-mutant peptides assemble and form fibrils at a much faster rate thanWT peptides. We identified five categories of intracellular aggregate— oligomers, single fibrils, fibril bundles, clusters, and aggresomes—that underline the heterogeneity of these A42 aggregates and represent the progression of A42 aggregation within the cell. Fluorescence-lifetime imaging (FLIM) and 3D structural illumination microscopy (SIM) showed that all aggregate species displayed highly compact structures with strong affinity between individual fibrils. We also found that aggregates formed by Arctic mutant A42 were more resistant to intracellular degradation than theirWT counterparts. Our findings uncover the structural basis of the progression of Arctic mutant A42 aggregation in the cell.