Johan Banér was born at Djursholm Castle in Uppland. As a four year old he was forced to witness how his father, the Privy Councillour Gustaf Banér, and uncle, Sten Axelsson Banér (also a Privy Councillour), were executed at the Linköping Bloodbath in 1600. They were accused of high treason by King Charles IX because of their support of King Sigismund. And though it was the father of King Gustavus Adolphus who had Banér's father executed the two men developed a strong friendship from an early age, mostly because Gustavus Adolphus reinstated the Banér family soon after his coronation.
In 1630, Gustavus Adolphus landed in Germany and 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 also Munich, and rendered conspicuous service at the Lech and at Donauwörth.
At the unsuccessful assault on Albrecht von Wallenstein's camp at the Alte Veste, Banér received a wound, and soon afterwards, when Gustavus marched towards Lützen, was left in command in the west, where he was opposed to the imperial general Johann von Aldringen. Two years later, as Swedish field marshal, Banér, with 16,000 men, entered Bohemia and, combining with the Saxon army, marched on Prague. But the complete defeat of Bernard 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, Carl Gustaf Wrangel and Lennart Torstenson at Kyritz and Wittstock (October 4, 1636), restored the paramount influence of Sweden in central Germany. However,the three combined armies were considerably 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 10 May 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's 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.