This study investigates the time-dependent behavior of sisal fibers under sustained uniaxial tensile loads in different environments, addressing gaps in understanding beyond quasi-static conditions. Testing involved immersion in water and alkaline solutions (0.5 M and 1 M NaOH) to simulate service conditions. Stress levels were set at 8, 16, and 33% of ultimate tensile strength (1, 2, and 4 N). Modifications, including alkaline treatment, styrene-butadiene polymer coating (50% and 100% v/v), and tannic acid functionalization, were applied to explore their impact on fiber creep behavior. Results indicated a logarithmic creep pattern, with a high constant rate in primary creep followed by secondary creep. Sisal fibers exhibited significant deformation under sustained load, and time-dependent strain varied with the applied load. High humidity increased the global rate of fiber deformation, resulting in elevated time-dependent strain levels. All treatments effectively reduced fiber creep strain, with tannic acid-functionalized fibers showing the best results. Subsequent quasi-static uniaxial tests assessed the mechanical response and durability of both pristine and modified fibers. Findings revealed that, compared to untreated fibers, modified sisal fibers demonstrated improved maintenance of mechanical properties after sustained-load tests in aggressive environments. This research contributes with valuable insights into the time-dependent behavior of sisal fibers and highlights the efficacy of various modifications in enhancing their durability under different loading conditions and environments.