Serving with the British army in Sierra Leone (1822), Laing was sent among the Mande people of the region by the governor, Charles (later Sir Charles) M'Carthy, to attempt to develop trade in goods and to abolish that in slaves. He also visited the capital of the Susu people, Falaba, now in Sierra Leone. In 1823–24 Laing fought in the war between the British and the Asante empire and returned to England with the news of M'Carthy's death in action.

His next mission was to visit Tombouctou and to explore the Niger River basin. In July 1825 he left the North African coast at Tripoli, Libya, on his journey across the Sahara. He reached Ghudāmis (Ghadames) in northern Fezzan, now in Libya, by September and then entered the vast country of the Tuareg. Before reaching Tombouctou on August 18, 1826, he had to fight for his life and was severely wounded. He left Tombouctou on September 24 and was murdered two days later. The journal of his earlier explorations, Travels in the Timannee, Kooranko, and Soolima Countries in Western Africa, was published in 1825.