He was the son of the bailiff Hans Rantzau (1452–1522) on Gut Neuhaus and Schmoel in today's Plön district and Margarethe Brockdorff (1477–19 August 1547).

His eldest brother was Melchior Rantzau (around 1496–1539), the most important statesman for foreign and financial policy in the duchies of Schleswig and Holstein.

Five other brothers of his also held important positions in the government of the duchies: Heinrich (or Henrik) (1501–1561) and Sievert († 1576) were ducal councilors and bailiffs, Jasper (or Caspar) († 1562) royal councilor and bailiff, Breide († 1562) royal governor of the duchies and Otto († 1585) provost of Uetersen monastery.

Balthasar Rantzau was kidnapped together with a noble boy at the beginning of August 1545 from his episcopal estate Kaltenhof on the Trave in the area of Alt-Lübeck not far from today's Bad Schwartau.

The culprit was the Mecklenburg nobleman Martin von Waldenfels, owner of Gut Gorlosen, who tried in this way to recover outstanding pay payments, which amounted to 1400 guilders, from King Christian III to blackmail from Denmark.

In 1543 he came to Kiel in the entourage of Prince Magnus of Mecklenburg for his marriage to Elisabeth, one of the king's sisters, where he repeated his claim.

Eventually, Governor Breide Rantzau, the bishop's brother, nullified the guilt in the king's name because Waldenfels could produce no written contract for the alleged enlistment of horsemen, and threatened him with a lawsuit for his "lies".

[3] The king was not willing – and probably not able – to pay the ransom, the Rantzau brothers saw the kidnapping as a matter of state and the cathedral chapter only wanted to participate if the others paid.

However, since only the approximate whereabouts of the abductee and the kidnapper were known, intermediaries were used to negotiate with the princes of Mecklenburg and Brandenburg, including the chancellor Andreas von Barby, who was to become Rantzau's successor in 1556.

However, since Waldenfels was under Moritz von Sachsen's personal protection, Elector Joachim opened the trial at the end of 1548 only against the Bredows and a few other suspects.