He was the Head of State of the National Assembly of the Batavian Republic from 17 April 1797 – 1 May 1797, and Commissioner-General of the Cape Colony during the Interregnum from 21 February 1803 – 25 September 1804, in accordance with the short-lived Treaty of Amiens.

After that he entered political life, holding the following positions: The Staatsbewind of the Batavian Republic, after the return of the Dutch Cape Colony under the terms of the Treaty of Amiens, resolved that the executive and legislative authority of the Cape Colony should be committed to a governor and a council of four members, of whom one at least should be a colonist, by birth or long residence.

With these principles as a basis, the task of drawing up a plan of administration was entrusted to De Mist, an advocate of high standing and a member of the council for the Asiatic possessions and establishments.

[2] The document prepared by De Mist gave such satisfaction that he was sent out to receive the colony from the English, install the Dutch officials, and make such regulations as he might find necessary.

A very able military officer and man of high moral worth – Lieutenant-General Jan Willem Janssens – was appointed governor and was also commander-in-chief of the garrison.

On 30 December, General Dundas issued a proclamation absolving the inhabitants of the colony from the oath of allegiance to His Britannic Majesty (George III) on and after 1 January 1803.

After a temporary withdrawal of the order to hand over control, at sunset on the evening of Sunday 20 February 1803 the English guards were relieved by Dutch soldiers.

[2] De Mist announced that after making himself acquainted with the condition of the county, he would prepare a charter, which required ratification by the States-General.

In July 1804 De Mist published a proclamation declaring that all religious societies that worshipped an Almighty Being were to enjoy equal protection under the law, and that no civil privileges were to be attached to any creed.