John Forbes was employed by the Horticultural Society of London (later the Royal Horticultural Society) and became an associate of the Linnean Society in 1822. In February that year he was sent by the Horticultural Society to accompany the hydrographic survey expedition to East Africa and the Indian Ocean, led by Captain W.F. Owen*, and kept a diary of his experiences. On the outward journey in HMS Leven he collected plants around Rio de Janeiro, which he sent to the Royal Botanical Gardens at Kew.
The expedition arrived at Simons Bay on 8 July and stayed to 17 September. During this time Forbes collected minerals, bulbs, seeds and living plants, and prepared bird skins and herbarium specimens. He also accompanied Lieutenant Vidal of the Leven on a hydrographic survey of the Cape Peninsula. On leaving the Cape the expedition called at Algoa Bay, where more material was collected. They reached Delagoa Bay (now Baia de Maputo) late in September. Forbes participated in a survey of parts of the bay and one of its estuaries, and made some ethnological observations. After a stay of two months they surveyed the Mozambique Channel and visited Madagascar and the Comores, returning to Delagoa Bay on 1 February 1823. However, many members of the expedition became ill from malaria and eventually a third of the men died from the disease.
The expedition sailed for the Cape on 15 March to recover, and along the way Forbes collected some plants at Algoa Bay. They remained at the Cape from early April to 21 July. Upon returning to Mozambique Forbes joined a small expedition (under Lieut. Browne and including assistant surgeon Kilpatrick) on a survey of the Zambesi River, starting at Quilimane on the north side of the delta in July 1823. He collected plants and birds, but died of malaria along the river and was buried at Vila de Sena. Browne and Kilpatrick also died of malaria soon afterwards. His notes were later published in Captain Owen's Narrative.... The genus Forbesia was named after him by C.F. Ecklon*, while he was also commemorated in the names of the species Amaryllis forbesii, Grewia forbesii and Loranthus forbesii.