He joined the Diplomatic Service and in 1803 was appointed ambassador to the Hanse Towns, minister-resident of Great Britain at Hamburg (at that time a "free city"), and chargé d'affaires to the Circle of Lower Saxony.
[2] In Berlin great indignation was expressed and the King of Prussia, as Protector of the Circle of Lower Saxony and guardian of the free cities, ordered his minister at Paris to demand Rumbold's release.
The next day, on the orders of Napoleon, he was conveyed to Cherbourg and put on board a French cutter, sailing under flag of truce, which delivered him to the British frigate Niobe, in which he arrived at Portsmouth.
l'Empereur est, que M. Rumbold soit saisi, et que l'on s'assure de tous les papiers qui seront trouvés dans la maison.
I invite you, in consequence, M. le Maréchal, to take all the necessary measures for the most prompt execution of this operation.Let the requisite communications be made as soon as the operation shall be terminated, and Rumbold, with all his correspondence, shall be within your lines.It will be superfluous to instruct, in this regard, the Diplomatic Agents.