While magnesium implants are already commercially available, exploring new manufacturing methods and alloy designs is essential to enhance their properties and clinical safety. This study assesses the in vitro and in vivo degradation of disc-shaped implants of the Mg-1Ca-0.5Zn-0.1Y-0.03Mn (at.%) alloy prepared by the rapidly solidified ribbon consolidation (RSRC) and hot extrusion of cast material (ingot metallurgy, IM). Electrochemical measurements and 28-day in vitro immersion tests were carried out under simulating physiological conditions. The cytotoxicity was evaluated by exposing L929 fibroblasts to RSRC Mg extracts. For the first time, the biocompatibility and degradation of the Mg-1Ca-0.5Zn-0.1Y-0.03Mn at.% alloy was assessed in a partial thickness calvaria defect model on male Wistar rats after 7 and 28 days of implantation. In vitro results showed that the RSRC alloy exhibited a more homogeneous microstructure and significantly lower corrosion rates compared to IM samples, with corrosion rates below 0.12 mm/year. The RSRC specimens exhibited lower pH and osmolality, and cytotoxicity assays confirmed that extracts from the RSRC alloy were non-toxic to L929 fibroblasts. In vivo, the RSRC alloys demonstrated slow degradation with corrosion rates remaining below 0.25 mm/year compared to pure Mg. Despite the presence of hydrogen gas cavities, new bone formation was observed suggesting that the RSRC alloy is a suitable material for the potential use as medical-grade absorbable Mg implants.