${\mathrm{Co}}_{2}$-based Heusler alloys show diverse magnetic behavior, extending from the localized Heisenberg to the delocalized itinerant, and thus cannot be generalized with one specific model. Therefore, we bring together experiments and first-principles calculations to reveal the origin of long-range ferromagnetism in the ${\mathrm{Co}}_{2}$-based Heusler alloy ${\mathrm{Co}}_{2}\mathrm{HfSn}$. The precise value of the critical exponents $\ensuremath{\beta}=0.471(4), \ensuremath{\gamma}=1.02(4)$, and $\ensuremath{\delta}=3.273(5)$ with the Curie temperature ($T{}_{c}=430$ K) were determined experimentally by means of different analytical methods such as modified Arrott plot analysis, the Kouvel-Fisher method, and critical isotherm analysis. The deduced critical exponents belong to the theoretical prediction of the three-dimensional mean-field model. The magnetic exchange distance is found to decay as $J(r)\ensuremath{\sim}{r}^{\ensuremath{-}4.52}$, indicating the long-range magnetic interaction in ${\mathrm{Co}}_{2}\mathrm{HfSn}$. Moreover, the magnetic entropy change $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}$ features a maximum at ${T}_{c}$, i.e., $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}^{max}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}$ 29.87(6) J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 5 T, whereas power law fitting of $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{M}^{max}$ with $H$ gives ${n}^{\ensuremath{'}}=0.663(5)$, also confirming the mean-field-type magnetic interaction in ${\mathrm{Co}}_{2}\mathrm{HfSn}$. Meanwhile, in order to explore the origin of long-range ferromagnetic ordering in ${\mathrm{Co}}_{2}\mathrm{HfSn}$, spin-polarized density functional theory calculations were employed in this study. No stable antiferromagnetic solution with stable local magnetic moments at the Co or Hf site was observed in our fixed spin moment simulation, revealing that ${\mathrm{Co}}_{2}\mathrm{HfSn}$ is a highly itinerant magnet with a long-range ferromagnetic solution together with robust half-metallic conduction stable against thermal excitation higher than room temperature. Furthermore, our theoretical analysis shows that ${\mathrm{Co}}_{2}\mathrm{HfSn}$ belongs to an unconventional family of itinerant magnetism. Both the 100% spin-polarized metallicity and ferromagnetism arise from the Co $d$ orbitals, causing it to be unique compared to the conventional itinerant family of Ni, Co, or Fe, where magnetism and metallicity originate from different orbitals characters.