From henceforth to the end of her life Biren’s influence over the duchess was paramount. On the elevation of Anna to the Russian throne in 1740, Biren, who had in the meantime married a Fräulein von Treiden, came to Moscow and received many honours and riches. At Anna's coronation (19 May 1730) he became grand-chamberlain, a count of the Empire, on which occasion he is said to have adopted the arms of the French ducal house of Biron, and was presented with an estate at Wenden with 50,000 crowns a year.
He soon made himself cordially detested by Russians of every class. He was not indeed the monster of iniquity he is popularly supposed to have been. His vices were rather of the sordid than of the satanic order. He had insinuating manners and could make himself very agreeable if be chose; but he was mean, treacherous, rapacious, suspicious and horribly vindictive. During the latter years of Anna’s reign, Biren increased enormously in power and riches. His apartments in the palace adjoined those of the empress, and his liveries, furnitures and equipages were scarcely less costly than hers. Half the bribes intended for the Russian court passed through his coffers. He bad landed estates everywhere. A special department of state looked after his brood mares and stallions. The magnificence of his plate astonished the French ambassador, and the diamonds of his duchess were the envy of princes.
The climax of this wondrous elevation occurred when, on the extinction of the line of Kettler, the estates of Courland, in June 1737, elected Biren their reigning duke. He was almost as much loathed in Courland as in Russia; but the will of the empress was the law of the land, and large sums of money, smuggled into Courland in the shape of bills payable in Amsterdam to bearer, speedily convinced the electors. On her death-bed Anna, very unwillingly and only at his urgent entreaty, appointed him regent during the minority of the baby emperor, Ivan VI of Russia. Her commonsense told her that the only way she could save the man she loved from the vengeance of his enemies after her death was to facilitate in time his descent from his untenable position. Finally, on 26 October 1740, a so-called "positive declaration" signed by 194 dignitaries, in the name of the Russian nation, conferred the regency on Biren.
Biren’s regency lasted exactly three weeks. At midnight on 19 November 1740 he was seized in his bedroom by his ancient rival, Field Marshal Munnich. The commission appointed to try his case condemned him (11 April 1741) to death by quartering, but this sentence was commuted by the clemency of the new regent, Anna Leopoldovna, the mother of Ivan VI, to banishment for life at Pelin in Siberia. All Biren’s vast property was confiscated, including his diamonds, worth £600,000.
For twenty-two years the ex-regent disappeared from the high places of history. He re-emerged for a brief moment in 1762, when the philo-German Peter III of Russia summoned him to court. He was now too old to be in any one’s way, and that, no doubt, was the reason why Catherine II of Russia re-established him (1763) in his duchy, which he bequeathed to his son Peter. Misfortune had chastened him, and the last years of his rule were just and even benevolent, if somewhat autocratic. He died at Mittau, his capital, on 28 December 1772.