Abstract: This study proposes to use the unconfined compressive strength (UCS) and the bender element (BE) tests for determining the strength and the initial small-strain shear modulus of Bangkok soft marine clay improved by cement and polyester fibers. This study varies the content of admixed cement (1%–20%) and polyester fibers (0–20%), including the curing time (3–28 d) for preparing 360 samples. Moreover, this study uses the Michaelis-Menten kinetics concept to model cement hydration saturation. From the study, it is concluded as follows. The modelled results reveals that at least 10% cement and 1% polyester fiber are recommended to attain the 28-d UCS standards (294 kPa) for highway subgrade materials in Thailand. This also fulfils sustainable construction due to reducing normal-use cement from 20% to 10%. Unfortunately, the addition of polyester fibers into the Bangkok clay with at least 5% cement reduces shear modulus by 1.12–1.32 times. The Abram's relationship between shear modulus and the mixing-water-to-cement ratio is found time-dependent. From the composite theory, the BE detects the polyester fiber zone as a defect in the Bangkok clay (matrix) with 5%–20% cement. So, the 28-d shear modulus in the polyester fiber zone is negative (up to −0.034 MPa for 20% fiber), similar to softening phenomenon in concrete cracking (negative stiffness). For the 28-d shear modulus of fiber zone, the optimum cement content is around 2% for the positive influences of polyester fibers. Experimentally, the time-dependent normalized UCS for 10% and 20% cement is compatible with other studies, and its development rate increases with the cement content as 0.3017, 0.3172 and 0.3204 for 5%, 10% and 20% cement, respectively. The 28-d relationship between shear modulus and UCS shows that low-cement soft clay requires high polyester fiber content (5%–20%) to activate UCS improvement. However, the soft clay with enough cement (20%) causes the uniformly distributed UCS improvement.