Abstract We present a strong lensing analysis of COOL J1241+2219, the brightest known gravitationally lensed galaxy at z ≥ 5, based on new multiband Hubble Space Telescope (HST) imaging data. The lensed galaxy has a redshift of z = 5.043, placing it shortly after the end of the “Epoch of Reionization,” and an AB magnitude z AB = 20.47 mag (Khullar et al.). As such, it serves as a touchstone for future research of that epoch. The high spatial resolution of HST reveals internal structure in the giant arc, from which we identify 15 constraints and construct a robust lens model. We use the lens model to extract the cluster mass and lensing magnification. We find that the mass enclosed within the Einstein radius of the z = 1.001 cluster lens is M ( < 5 .″ 77 ) = 1.079 − 0.007 + 0.023 × 10 13 M ☉ , significantly lower than other known strong lensing clusters at its redshift. The average magnification of the giant arc is 〈 μ arc 〉 = 76 − 20 + 40 , a factor of 2.4 − 0.7 + 1.4 greater than previously estimated from ground-based data; the flux-weighted average magnification is 〈 μ arc 〉 = 92 − 31 + 37 . We update the current measurements of the stellar mass and star formation rate (SFR) of the source for the revised magnification to log ( M ⋆ / M ⊙ ) = 9.7 ± 0.3 and SFR = 10.3 − 4.4 + 7.0 M ⊙ yr −1 , respectively. The powerful lensing magnification acting upon COOL J1241+2219 resolves the source and enables future studies of the properties of its star formation on a clump-by-clump basis. The lensing analysis presented here will support upcoming multiwavelength characterization with HST and JWST data of the stellar mass assembly and physical properties of this high-redshift lensed galaxy.