She was the eldest of the thirteen children of Frederick Louis, Duke of Schleswig-Holstein-Sonderburg-Beck, by his wife, Princess Luise Charlotte of Schleswig-Holstein-Sonderburg-Augustenburg (1658-1740).
Of all her siblings, only seven survived to adulthood: Frederick William II, who inherited Beck after succeeding his father; Charles Louis, later husband of the Countess Orzelska and ruler of Beck after the death of his nephew; Philipp Wilhelm, who died unmarried in 1729; Luise Albertine, by marriage von Seeguth-Stanislawsky; Peter August, who years later inherited Beck from his older brother; Sophie Henriette, Burgravine and Countess of Dohna-Schlobitten; and Charlotte, Abbess of Quedlinburg.
Dorothea married Georg Frederick Karl of Brandenburg-Bayreuth-Kulmbach, later Margrave of Bayreuth, on 17 April 1709 in Reinfeld and settled at Weferlingen.
In 1751 the 66-years-old Dorothea von Zeidewitz was sent to live under the care of the Swedish Count Nils Julius von Lewenhaupt, chief steward at the Bayreuth court, on whose estate Stäflö near Kalmar remained for the rest of her life, apparently on the instructions or with the approval of her son, Margrave Frederick of Brandenburg-Bayreuth, who also increased their annual allowance to §1000.
However, the Margrave adhered to the harsh instructions of his father's will of 1735, according to which his mother was never again allowed to step on Bayreuth soil.