Cosimo I de ' Medici ( 12 June 1519 â `` 21 April 1574 ) was Duke of Florence from 1537 to 1574 and then the first Medici Grand Duke of Tuscany from 1569 . Cosimo I de ' Medici ( 12 June 1519 -- 21 April 1574 ) was Duke of Florence from 1537 to 1574 , reigning as the first Grand Duke of Tuscany from 1569 .