Entering the Swedish army, he served with distinction in the wars with Russia and Poland, and had reached high rank when, in 1630, Gustavus Adolphus landed in Germany. As one of the king's chief subordinates, Banér served in the campaign of north Germany, and at the first Battle of Breitenfeld he led the right wing of Swedish cavalry. He was present at the taking of Augsburg and of Munich, and rendered conspicuous service at the Lech and at Donauworth.
At the unsuccessful assault on Wallenstein'ss camp at the Alte Veste, Banér received a wound, and, soon afterwards, when Gustavus marched towards Lützen, his general was left in command in the west, where he was opposed to the imperial general Aidringer. Two years later, as Swedish field-marshal, Banér, with 16,000 men, entered Bohemia, and, combined with the Saxon army, marched on Prague. But the complete defeat of Bernhard of Saxe-Weimar in the first Battle of Nördlingen stopped his victorious advance.
After this event the peace of Prague placed the Swedish army in a very precarious position, but the victories won by the united forces of Banér, Wrangel and Lennart Torstenson, at Kyritz and Wittstock (October 4, 1636), restored the paramount influence of Sweden in central Germany. Even the three combined armies, however, were decidedly inferior in force to those they defeated, and in 1637 Banér was completely unable to make headway against the enemy. Rescuing with great difficulty the beleaguered garrison of Torgau, he retreated beyond the Oder into Pomerania.
In 1639, however, he again overran northern Germany, defeated the Saxons at Chemnitz and invaded Bohemia itself. The winter of 1640-1641 Banér spent in the west. His last achievement was an audacious coup-de-main on the Danube. Breaking camp in mid-winter (a very rare event in the 17th century) he united with the French under the comte de Guébriant and surprised Regensburg, where the diet was sitting. Only the break-up of the ice prevented the capture of the place. Banér thereupon had to retreat to Halberstadt. Here, on May 10, 1641, he died, after designating Torstenson as his successor. He was much beloved by his men, who bore his body with them on the field of Wolfenbüttel. Banér was regarded as the best of Gustavus'ss generals, and tempting offers (which he refused) were made him by the emperor to induce him to enter his service. His son received the dignity of count.