As a four-year-old he was forced to witness the execution of his father, the Privy Councillour Gustaf Banér, and uncle, Sten Axelsson Banér (also a Privy Councillour), at the Linköping Bloodbath in 1600, due to accusations of high treason by King Charles IX because of their support of King Sigismund.

[citation needed] In 1630, Gustavus Adolphus landed in Germany to assist German Protestant forces in the Thirty Years War, and, as one of the king's chief subordinates, Banér served in the campaign of north Germany, including leading the right wing of Swedish cavalry at the first Battle of Breitenfeld.

Two years later, as Swedish field marshal, Banér, with 16,000 men, entered Bohemia and, combining with the Saxon army, marched on Prague.

[1] 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 and Alexander Leslie at Wittstock (4 October 1636), restored the paramount influence of Sweden in central Germany.

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.

Here, on 10 May 1641, he died, possibly due to advanced liver cirrhosis caused by his excessive alcohol consumption,[4] after designating Lennart Torstenson as his successor.

[1] On the other hand, the enemies of Sweden rejoiced and a parody requiem hoping that he was burning in hell was even composed in Bohemia, which had been badly pillaged by Banér.