Frederick William I (1562-1602) and John III (1570-1605) were the eldest sons of Duke John William I of Saxe-Weimar from the House of Wettin and his wife Princess Dorothea Susanne of the Palatinate. When the father of the two died in 1573, both sons were still minors. Contrary to the will, the Saxon Elector Augustus took over the regency, because of his close family ties.
In 1583 Frederick William reached the age of majority, but ruled independently only from 1586, after Augustus had died. His brother John was involved in the government from 1591 at the latest, because in this year Frederick William assumed regency over the Electorate of Saxony and spent most of his time there. The Elector Christian I had left only a minor son: Christian II. Frederick William led a lavish lifestyle, which he financed from the coffers of his protegé.
In 1601 he returned to Weimar and died the following year. His brother John rose to become the only Duke and ruled until his death in 1605. He entered the annals of the dynasty of Saxe-Weimar, mainly because eight of his eleven sons reached adulthood, the famous Eight Brothers of Saxony.