By 1620, Anstruther was James VI and I's main negotiator in Germany for a settlement on behalf of Frederick V and Elizabeth Stuart.
The following year, Anstruther was a gentleman of the Privy Chamber and served as a translator for the Imperial Ambassador at the English Parliament.
Although he continued as ambassador to Denmark-Norway after the coronation of Charles I, he left the Danish court in 1629 after the Treaty of Lübeck, finishing much of his residency from Hamburg.
In August 1630, Anstruther served as ambassador to the Diet of Regensburg, and was kindly received by Ferdinand II, Holy Roman Emperor.
Although the Diet was dissolved, Anstruther managed to secure another meeting with Ferdinand, before returning to Scandinavia, believing that any further work in the Holy Roman Empire would be prejudicial to Stuart interests.
These centered on trying to get the Duke of Bavaria elected as Emperor, and although the Dutch expressed reservations, Anstruther attended the Diet of Allied Princes at Heilbronn.
However, the Peace of Prague (1635) largely undermined Stuart interests in Germany, and in 1638 he turned down an appointment to represent Britain in a meeting with Christian IV in Hamburg.
[7] It is said that the painter and tapestry designer Francis Cleyn, who was in the service of Christian IV of Denmark, came to England in 1623 with Anstruther's recommendation to Prince Charles.
[8] The inventory of the art-works in the palaces of Charles I made by Abraham van der Doort records paintings brought home by Robert Anstruther.
These include a picture of a family brought from Germany (in 1630) and a "Martyrdom of St Bartolomew" attributed by Van der Doort to the Carracci in the King's chair room at Whitehall.