Functionally graded materials (FGMs) replace the discrete interfaces of multi-material additive manufacturing with continuous compositional gradients, mitigating delamination and cracking, but reported systems for FGM direct ink writing (DIW) rely on specialized hardware that limits reproduction by other laboratories. This work presents the design, analysis, fabrication, and validation of a low-cost, modular active-mixing extrusion system for functionally graded DIW on a gantry platform. Two syringe pumps mounted on a stationary frame deliver feedstocks through flexible tubing to a gantry-mounted chamber, where a helical ribbon impeller and an extrusion screw on a single motor-driven shaft mix and deposit the material; all critical components are additively manufactured, including a transparent mixing chamber and a 316L stainless steel impeller. The system carries 0.464 kg on the gantry and uses 70 parts. Analytical calculations confirmed laminar delivery flow, Herschel–Bulkley computational fluid dynamics guided the extrusion screw geometry to a 0.01 mL/s target flow rate, and finite element analysis confirmed factors of safety of 11.8 and above 400 for the impeller and frame. Extrusion calibration showed a linear actuator-to-mass response within ±10%, and printing with two dyed kaolin clay slips produced visually smooth color gradients, demonstrating an accessible, reproducible platform for FGM-DIW research with clay-based feedstocks; validation with compositionally distinct ceramic and metal slurries and quantitative gradient characterization remain future work.