Itaconic acid (IA) is a sustainable platform chemical with growing industrial relevance, produced mainly by Aspergillus terreus under controlled nutrient conditions. This study evaluates the effects of manganese (Mn²⁺) and phosphate (PO₄³⁻) on IA production from glucose in submerged fermentation. Under trace Mn²⁺ conditions, IA reached 66.52 g/L at 192 h with an 84.76% molar yield, indicating highly efficient carbon conversion. In contrast, excess Mn²⁺ (30–200 μg/L) reduced IA titers to 50 g/L and promoted increased biomass formation and dispersed hyphal morphology. Phosphate supplementation alone also decreased IA yield by shifting metabolism toward biomass, while combined Mn²⁺ PO₄³⁻ treatment caused an almost complete loss of IA production with dry cell weight reaching 40 g/L. These results demonstrate that only under trace Mn²⁺, controlled phosphate levels, and proper fungal morphology can IA production reach its maximum potential, providing valuable guidance for industrial fermentation optimization.