Cellulose ethers are used as additives in render mortars to improve their water retention and sag resistance, and they are also used in special concrete applications like self-compacting and 3D-printable concrete as a rheology modifier. This study explores the effect of hydroxypropyl methylcellulose (HPMC) on the early-age hydration kinetics and pore structure of cement paste. The hydration kinetics was studied by performing isothermal calorimetry for a 7-day period with a cement paste of 0.45 water-cement ratio having HPMC dosage of 0%, 0.1%, and 0.2% of cement weight. The hardened cement paste’s pore structure was characterized using the mercury intrusion porosimetry technique. In addition, the 7-day compressive strength studies were also determined using mortar cubes. From the isothermal studies, although the dormant period and the initial heat peak got delayed, the cumulative heat at the end of 7 days was found to be marginally higher with an increase in the percentage of HPMC addition. HPMC can quickly dissolve in water and increase the viscosity of the liquid phases, thus decreasing the movement of calcium, sulfate, and hydroxyl ions, delaying the dormant period. The MIP studies revealed an increase in the porosity with an increase in HPMC dosage, possibly due to entrapment of air voids caused by HPMC addition. The increase in porosity also caused a significant decrease in compressive strength with an increase in the HPMC dosage. This study helps understand the detrimental effects of HPMC addition, particularly when they are to be used at higher dosages, on the pore structure and the mechanical properties of the cementitious systems.