Miniaturizing electronic components calls for controlled synthesis of conductive metal nanoparticles with tunable and narrow size distributions. This has become increasingly essential, yet a significant challenge for the use as conductive inks or pastes in various applications, including multilayer ceramic capacitors, flexible electronics, and magnetofluids. In this work, we report the controllable synthesis of uniform spherical nickel nanoparticles via a mild aqueous-phase reduction strategy using triethanolamine (TEA) as a modulating agent. Through strategic control of both the dosage and the timing of TEA addition, we successfully decoupled nucleation and growth stages, enabling precise particle size regulation within the 95–270 nm range. Moreover, electrical conductivity tests on both pressed Ni pellets and printed ink lines confirmed the inherited metallic nature. The correlation between particle size and conductivity can be attributed to enhanced interparticle neck formation during sintering, which reduces electrical resistance. Both magnetic saturation (M_s) and remanent magnetization (M_r) are also positively related to particle size, while coercivity (H_c) shows a converse relationship, consistent with a size-dependent transition from surface-disordered to bulk-like multidomain magnetic behavior. This study not only provides an environmentally benign and scalable strategy for size-tunable Ni nanoparticle synthesis but also offers new promises for their integration in next-generation electronic and magnetic devices.