CoCrNi Medium Entropy Alloys (MEAs) are the object of several studies due to their interesting strength/ductility balance, with yield strength up to 1 GPa and a fracture 70 % respectively, originated by its single-phase FCC atomic structure [1]. Recently, CoCrNi has also been synthetized in thin film form reporting large hardness and elastic modulus up to 10 GPa and 250 GPa respectively, due to a peculiar nanotwinned structure and columnar grains with coherent twin boundaries [2]. Similarly, the addition of Fe to form FexCoCrNi(1-x) led to an enhancement of the mechanical behavior due to the stabilization of the FCC phase and the formation of a Cr-rich σ phase [3] with a small grain size of ~20 nm achieving high hardness > 11 GPa. [4] However, the research on MEA-TFs is still in its early stages especially concerning the relationship atomic-microstructure and mechanical behavior, while the film growth mechanisms of such compositionally complex films are still poorly explored. Here, we deposited CoCrNi and FexCoCrNi thin films with controlled thicknesses from 15 nm up to 700 nm, and compositions with Fe content varying from 15 up to 33 % at. The 700 nm thick CoCrNi film (Fig. 1a) has an average grain size of ~26 nm and a (111) oriented FCC structure. Nanoindentation and optoacoustic techniques report high hardness and elastic modulus up to 7.8 and 196 GPa, respectively. Moreover, the investigation of the early stage growth regimes by Selected Area Electron Diffraction-Transmission Electron Microscopy (SAED-TEM) reveals a transition from HCP (0001) (Fig.1 b) to mixed FCC/amorphous (Fig. 1c) for thicknesses from 15 up to 40 nm. This can be related to the low stacking fault energy of the alloy (∼25 mJ m−2). Finally, we show that Fex(CoCrNi)1-x films have an FCC phase structure with a preferred (111) orientation which is kept even for 33% at. Fe without phase segregation, while the presence of planar defects such as twins as well as the large lattice distortion is responsible for high hardness and elastic modulus values up to 10 and 250 GPa, respectively. Overall, our study shed light on structure–mechanical properties of CoCrNi and FexCoCrNi(1-x) HEA-TFs with potential impacts in the field of hard coatings and microelectronics. [1] Gludovatz. B, et al. Nat Commun 7, 10602 (2016) [2] F. Cao, et al., Surface and Coatings Technology, Volume 335, 2018 [3] M.K. Kini et al., Surface & Coatings Technology 410 (2021) [4] Nagy, P et al., Materials 2022, 15, 2319