A facile, postsynthetic treatment of a designed composite of pyrimidine-based porous-organic polymer and graphene (PyPOP@G) with ionic Pt, and the subsequent uniform electrodeposition of Pt metallic within the pores, led to the formation of a composite material (PyPOP-Pt@G). The pyrimidine porous-organic polymer (PyPOP) was selected because of the abundant Lewis-base binding sites within its backbone, to be combined with graphene to produce the PyPOP@G composite that was shown to uptake Pt ions simply upon brief incubation in H2PtCl6 solution in acetonitrile. The XPS analysis of PyPOP@G sample impregnated with Pt ions confirmed the presence of Pt(II/IV) species and did not show any signs of metallic nanoparticles, as further confirmed by transmission electron microscopy. Immediately upon electrochemical reduction of the Pt(II/IV), metallic Pt (most likely atomistic Pt) was observed. This approach stands out, as compared to Pt monolayer deposition techniques atop metal foams, or a recently reported atomic layer deposition (ALD), as a way of depositing submonolayer coverage of precious catalysts within the 1-10 nm pores found in microporous solids. The prepared catalyst platform demonstrated large current density (100 mA/cm2) at 122 mV applied overpotential for the hydrogen evolution reaction (HER), with measured Faradaic efficiency of 97(±1)%. Its mass activity (1.13 A/mgPt) surpasses that of commercial Pt/C (0.38 A/mgPt) at the overpotential of 100 mV. High durability has been assessed by cyclic and linear sweep voltammetry, as well as controlled potential electrolysis techniques. The Tafel plot for the catalyst demonstrated a slope of 37 mV/decade, indicating a Heyrovsky-type rate-limiting step in the observed HER.