Rehnskiöld was born in Stralsund as a son of Gerd Antoni Rehnskiöld and Birgitta Torskeskål. During his life, he played an important role in the Great Northern War. He commanded at the Battle of Fraustadt, after which King Karl XII made him a Field Marshal and Count. Highlights of his career include a highly successful campaign against Poland from 1701-1703, and the disastrous Ukrainian campaign which ended with the Battle of Poltava in 1709. The Battle was a disaster for the Swedish army and Rehnskiöld was captured by the Russians. He remained in Russian captivity until 1718.