Protein based composites, such as nacre and bone, show astounding evolutionary capabilities, including tunable physical properties. Inspired by natural composites, we studied assembly of atomistically thin inorganic sheets with genetically engineered polymeric proteins to achieve mechanically compliant and ultra-tough materials. Although bare inorganic nanosheets are brittle, we designed flexible composites with proteins, which are insensitive to flaws due to critical structural length scale (∼2 nm). These proteins, inspired by squid ring teeth, adhere to inorganic sheets via secondary structures (i.e., β-sheets and α-helices), which is essential for producing high stretchability (59 ± 1% fracture strain) and toughness (54.8 ± 2 MJ/m ³ ). We find that the mechanical properties can be optimized by adjusting the protein molecular weight and tandem repetition. These exceptional mechanical responses greatly exceed the current state-of-the-art stretchability for layered composites by over a factor of three, demonstrating the promise of engineering materials with reconfigurable physical properties.