Metastability engineering opens a new avenue to design high-entropy alloys (HEAs) originally proposed to benefit from phase stabilization. Meanwhile, boundary engineering via embedding planar defects such as stacking faults and nanotwins into the matrix of metals provides them with unique mechanical properties. In this work, for the first time, we combine the above two strategies to prepare Al0.1CoCrFeNi HEA pillars populated with a high density of stacking faults and nanotwins. It is uncovered that the stacking faulted (SF) Al0.1CoCrFeNi HEA pillars manifest ultrahigh strength exceeding 4.0 GPa and considerable compressive plasticity over 15%, much superior to their nanotwinned (NT) counterparts. Compared with the nanotwins undergoing detwinning during plastic deformation, the stacking faults in Al0.1CoCrFeNi high-entropy alloy thin films (HEAFs) are quite stable to hinder dislocation motion. Our findings not only endow the Al0.1CoCrFeNi HEAs with a predominant combination of strength and compression deformability, but also shed light on a new perspective for overcoming the strength and ductility trade-off in structural materials.