The gas sensing performance of monolayer Hf2CO was evaluated by means of first-principles calculation. Present results indicate that Hf2CO sheet possesses high selectivity and remarkable sensitivity toward NH3 molecule, by virtue of the moderate adsorption energy, sharply enhanced electronic conductivity beyond a specific bias voltage and appreciable work function change, as well as good stability, short recovery time. Moreover, the interaction of NH3 with Hf2CO can be enhanced by pre-adsorbed H2O molecule via forming hydrogen bonds between the adsorbates, while pre-covered O-2 molecule can barely affect the adsorption of NH3 molecule. Particularly, NH3 capture/release on monolayer Hf2CO is predicted to be realized by applying external strains and/or electric fields. These appealing features signify monolayer Hf2CO as promising nanomaterials for gaseous NH3 detection and storage.