To reduce the environmental impact of cement production and save energy, high-filler low-water (HFLW) systems are a promising solution. These systems depend on low water-to-binder ratios and the use of supplementary cementitious materials (SCMs) to significantly reduce the clinker factor. As for other systems with SCMs reacting less rapidly than ordinary Portland cement (OPC), strength development at an early age may be a challenge for HFLW systems when further implemented in concrete applications. This study evaluated mortar mixes with an HFLW binder composed of ordinary Portland cement (OPC), limestone filler, and ground granulated blast furnace slag (GGBS), with varying water-to-binder ratios (W/B). Selected mixes were further optimized by adding calcium-silicate-hydrate (C-S-H) seeding at 2%, 3%, and 5% by binder weight. The results show that C-S-H seeding improved the early age compressive strength, with a 95% strength increase after 1 day and a 45% increase after 3 days compared to the samples without C-S-H seeding. Different methods for the application of C-S-H seeding were also investigated, and it was observed that mixing the C-S-H with the mixing water and using a high shear mixer was the most efficient way to introduce the C-S-H to the mix. The approach was upscaled to the concrete scale, and the mix with 3% C-S-H seeding demonstrated the same strength improvement of 95% after 1 day, achieving a strength of 43 MPa after 7 days, along with the required fresh properties and air content. This research strengthened the interest of C-S-H seeding addition to improve HFLW binders’ early age strength, enhancing the ability to design binders with high clinker reduction without losing critical properties.