Information related to the geological formation processes, degree of evolution of certain fluids and rocks, origin, type and migration of crude oil is imprinted in the isotopic composition of certain proxy elements in crude oil. Mg has been used as isotopic proxy in geochemical, environmental and/or biomedical applications. The isotopic analysis of Mg has been performed in many matrices already, including environmental samples, geological and archeological materials, biological fluids and tissues. However, no papers have reported Mg isotopic analysis of crude oil so far. As a result, information on the isotopic composition of this element in crude oil is still lacking. Thus, the goal of the present work was to develop a method for Mg isotopic analysis of medium crude oil using high-precision multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) to provide the community with analytical tools for oil provenancing and tracing of the oil deposit formation. Two sample preparation methods were evaluated: (i) microwave-assisted wet digestion with a pressurized digestion cavity (MAWD-PDC); and (ii) solubilization of inorganic solids after the ASTM D4807 test method. Using MAWD-PDC, up to 0.5 g of crude oil was efficiently digested using just 6 mL of 14.4 mol L-1 HNO3 (75 min, temperature up to 250 °C). On the other hand, the ASTM based method does not require any sophisticated equipment or the use of halogenated solvents. With this method up to 10 g of oil can be dissolved in toluene and filtered through a nylon membrane. Elements present as inorganic solids, such as Mg, are retained on the nylon membrane and can be easily recovered in water. Isolation protocols were fine-tuned to the composition of crude oil solutions obtained upon MAWD-PDC and solubilization. Mg isolation was successfully carried out using the cation exchange resin AG 50W-X8. Both sample preparation methods were shown to be suitable for subsequent determination of Mg concentration and isotope ratio. A proof-of-concept study showed that the Mg isotopic composition of the Brazilian crude oils was within the range observed for seawater and the deposit bedrock, suggesting that the methods developed can be considered as promising tools to decipher the formation history of oil reservoirs.