Multi-principal element alloys have attracted significant attention of materials scientists because some of them have demonstrated outstanding strength and plasticity. For example, the CrCoNi medium-entropy alloy with the equiatomic concentration shows superior yield strength and ductility of ~ 1.3 GPa and 40%, respectively. The strength of the alloy could be further increased by reducing the grain size down to tens of nanometers. Here we used the magnetron sputtering technique to produce a CrCoNi alloy film with a thickness of 5 µm on a copper substrate. The film is of a columnar grain structure with an average grain size of 19 nm. Pillars with a diameter of 1 µm and a length of 2 µm and with their axial direction parallel or perpendicular to the columnar direction were prepared using the focused ion-beam technique. In-situ compression scanning electron microscopy (SEM) was used to explore the mechanical properties and deformation mechanisms of the pillars. Transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) were used for detailed micro-structural characterization before deformation, at yield and after failure. All pillars presented yield strength of > 4 GPa and engineering strain of > 40%. Mechanisms responsible the extraordinary mechanical properties are explored based on the detailed structural characterization.