Quesnel joined Jean-Andoche Junot's army for the 1807 Invasion of Portugal. He was named governor of Porto (Oporto) and Entre-Douro-e-Minho Province, with authority over the Spanish troops occupying the region. On 6 June 1808 news of the Dos de Mayo Uprising reached Porto and Spanish General Belesta seized Quesnel, his staff, and his 30-man dragoon escort as prisoners. After urging the Portuguese to revolt against the French, Belesta immediately marched back to Spain with his 6,000 soldiers and his prisoners. Quesnel was handed over to the British at Corunna, but was freed when French soldiers captured the city on 16 January 1809.