 There, Roos trained with his brother Johann Heinrich Roos during the years (1647–51) in history painting with Guilliam Dujardin (1597–after 1647), in landscape with Cornelis de Bie and in portraiture with Barent Graat.The brothers returned with the rest of the family to Germany in 1653, where they worked for a monastery in Mainz.When Johann left, Theodor Roos won a commission to paint the civic guard in Mannheim.