Transforming cellulosic materials into activated carbon (AC) offers a promising route to valorize biowastes for applications requiring precise pore structures such as supercapacitors, gas separation, and storage. Yet, the impact of the precursor's structure on chemical activation with KOH remains insufficiently understood, limiting the reproducibility and optimization of AC properties. Systematic studies exploring how varying the structural properties of cellulosic precursors affect activation outcomes are lacking. Here, we demonstrate that incorporating regenerated cellulose microspheres (CMS) into a pulp fiber matrix significantly enhances KOH impregnation, yielding improved AC characteristics, including increased specific surface area (SSA) and optimized pore size distribution (PSD). WAXS and FTIR analyses confirmed that higher CMS content increases the amorphous regions within the semi-crystalline precursor. EDX mapping further showed superior KOH penetration into CMS-rich regions compared to the more limited interaction with pulp fibers. Consequently, TEM, SAXS, and N₂ adsorption analyses revealed that higher CMS content improves microporosity and increases SSA. Additionally, thermogravimetric analysis revealed that an uneven distribution of reactive, less crystalline CMS regions leads to localized over-activation, resulting in distinct changes in porosity. These findings demonstrate that the heterogeneous distribution of domains with differing crystallinity, typical of lignocellulosic materials, influences activation behavior and may partly explain the variability often observed in biomass-derived ACs. Our work establishes a clear mechanistic link from precursor structure to KOH accessibility and final pore architecture, demonstrating that precursor-structure engineering can effectively optimize AC properties and provides an often overlooked alternative to harsh activation conditions.