Stretchable conductive films were obtained by screen printing and thermal treatment of a homogenous ink comprising a thermally reducible silver formate complex, an acrylate monomer, and a radical initiator. In the curing process, both the filler nanoparticles and the polymer matrix are generated in situ, at temperatures as low as 100 °C. The obtained conductors, consisting of percolated silver nanoparticles embedded in a polymeric matrix, typically show a resistivity of (2–4) × 10–5 Ω·m. When applied on an elastomeric substrate, the composite is stretchable up to 200% with very low R/R0 values, which is unprecedented for stretchable silver composite inks. Quasi-in situ confocal laser scanning microscopy of the strained samples revealed an initial fracture strain above 40%, which is unusually high for metal–nanoparticle films. The described system was compared to some commercial stretchable screen-printing inks and proved superior with regard to both R/R0 and resistance to cyclic tensile loading.