Nanosilica and polymers are both well-known admixtures of cementitious mixtures, changing their properties differently upon employment. However, having them incorporated simultaneously, and their possible interactions in the mixtures, is yet to be studied. In this research, mortar mixtures, modified by one type of polymer latex (Styrene butadiene acetate) and two types of nanosilica colloids (different average particle sizes), were investigated for their flowability and long-term mechanical properties. Interesting observations were made in both fresh and hardened state; in the fresh state, the high water demand of nanosilica incorporating mixtures was moderated at the presence of the polymer, and in the hardened state, improvements were observed, especially in terms of flexural strength. Mortar mixtures were produced with different amounts of polymer and colloidal nanosilica, and, accordingly, numerical analyses and simulations have been conducted to model and gain a better understanding of the investigated properties.