One of his elder brothers Nagayoshi was killed in the Battle of Komaki and Nagakute in 1584, and later Tadamasa succeeded to the Mori estate as his other elder brothers had died earlier as well.
天正12年（1584年）、兄の長可が小牧・長久手の戦いで戦死し、既に他の兄たちも早世していたため、家督を相続。