At the age of fifteen he became one of the pages of the young King Gustavus Adolphus and served during the Prussian campaigns of 1628 - 1629. In 1629 he was set over the Swedish artillery, which under his guidance materially contributed to the victories of Breitenfeld and Lech. The same year he was taken prisoner at Alte Veste and shut up for nearly a year at Ingolstadt. Under Johan Banér he rendered distinguished service at the Battle of Wittstock and during the energetic defence of Pomerania in 1637 - 1638, as well as at the Battle of Chemnitz and in the raid into Bohemia in 1639. Illness compelled him to return to Sweden in 1641, when he was made a member of the Privy Council.
The sudden death of Banér in May 1641 recalled Torstenson to Germany as generalissimo of the Swedish forces and Governor General of Pomerania. He was at the same time promoted to the rank of Field Marshal. In 1642 he marched through Brandenburg and Silesia into Moravia, taking all the principal fortresses on his way. On returning through Saxony he well nigh annihilated the imperialist army at the second Battle of Breitenfeld on October 23, 1642. In 1643 he invaded Moravia for the second time, but was suddenly recalled to invade Denmark, when his rapid and unexpected intervention paralysed the Danish defence on the land side, though Torstenson's own position in Jutland was for a time precarious owing to the skilful handling of the Danish fleet by Christian IV of Denmark. In 1644 he led his army for the third time into the heart of Germany and routed the imperialists at Jiiterbog November 23. At the beginning of November 1645 he broke into Bohemia, and the victory of Jankau on February 24, 1645 laid open before him the road to Vienna. Yet, though one end of the Danube bridge actually fell into his hands, his exhausted army was unable to penetrate any further and, in December the same year, Torstenson, crippled by gout, was forced to resign his command and return to Sweden. In 1647 he was created a count. From 1648 to 1651 he ruled all the western provinces of Sweden, as governor-general. On his death at Stockholm on April 7, 1651 he was buried solemnly in the Riddarholm Church, the Pantheon of Sweden. Torstenson was remarkable for the extraordinary and incalculable rapidity of his movements, though very frequently he had to lead the army in a litter, as his bodily infirmities would not permit him to mount his horse. He was also the most scientific artillery officer and the best and most successful engineer in the Swedish army.