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. In 1631, after a sojourn to London, Anstruther returned to Hamburg. En route he visited The Hague to discuss new proposals offered by Charles I. 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. In 1633, he attended another diet of Protestant princes and continued negotiating in Germany through April 1635. 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. This may have been due to the signing of the National Covenant and the outbreak of the Bishops' Wars in Scotland. He returned as an envoy to Denmark on behalf of both Charles I and the English parliament in 1641, and signed the Solemn League and Covenant in 1643. He died by late 1644 and was buried at Westminster Abbey on 9 January 1645.