We report the development of a temporally stable in-fiber electric field sensor using electro-optic (EO) polymer. The design consists of partially replacing the core of a D-fiber with AJL8/APC EO polymer resulting in a device insertion loss less than 2 dB. The design method is presented for the optimization of the hybrid waveguide. The resulting design is demonstrated to have an Epi of 134.6 MV/m at a modulation frequency of 990 MHz, which exhibited no appreciable temporal decay over a time span of over 200 hours and a sensitivity of 7.76 V/(m sqrt(Hz)). The design of the sensor packaging is also presented, which has an outer diameter of 1.5 mm. Mechanical tests demonstrate that the sensor withstands 17 rotations, a bending radius of 0.11 cm, and a tensile force of 4.5 N, confirming that the sensors are mechanically robust.