Mark Johnston McKen received some training in horticulture and from 1840 to 1849 gained considerable experience in the growing of sugar cane on the Golden Grove Sugar Estate near Port Morant, Jamaica. He was also for some time in charge of the Bath Botanic Gardens in Jamaica. After returning to Scotland in 1850 he came to Natal in October that year and settled in Durban. He brought 29 species of tropical crop plants with him from the Royal Botanical Gardens at Kew, near London, including mangoes, breadfruit, ginger, coffee, arrowroot, camphor, pawpaw, guava, cocoa, cinnamon, blood orange and black pepper. These were handed over to the Natal Agricultural and Horticultural Society (established in 1848), which elected him on their committee. The society began to develop a botanical garden on a 10 hectare site on the Berea to test out and acclimatise crop and horticultural plants and in June 1851 appointed McKen as its curator. He received fifty pounds per year, a hut to live in, and was allowed excess produce from the garden. His efforts were concentrated on growing and distributing species suitable for commercial planting, but he collected plants in his spare time. The society was able to hold its first show on 1 August 1851, in its own grounds, although the development of the garden was hampered by a chronic shortage of funds. In June 1852 McKen described the manufacture of the first sugar in the colony, at the estate of Mr Edmund Morewood, and concluded that sugar could be produced profitably in Natal. Thus he did much for the economic development of the colony. He resigned as curator in July 1853 to become manager of the sugar estate of Messrs Chiappini and Co. and later that of James R. Saunders, both at Tongaat, and is regarded as one of the pioneers of Natal's sugar industry. In his new post he made trading trips into Zululand during which he collected natural history specimens for commercial purposes. He encouraged Katherine Saunders* in her early collection and painting activities, and in 1861 married Margaret Wood, sister of the botanist John M. Wood*. They had six surviving children, including a son who was named Mark Johnston like his father.
In December 1860 McKen was again appointed curator of the Durban Botanic Gardens. Between 1862 and 1865 he made one or two collecting trips each year, some of them with William T. Gerrard* in the Tugela basin and in Zululand. Their specimens, including many novelties, were acknowledged by W.H. Harvey* in the prefaces to Volumes 2 and 3 of the Flora Capensis(1862, 1865). Most of their original plant specimens were sent to the Royal Botanical Gardens at Kew, in England, but duplicates remained in Natal and, with McKen's own collections, later formed the nucleus of the Natal Herbarium (established by John M. Wood in 1882). Many of his specimens were new to science, though he collected mainly within some 30 km of Durban. Kew received both herbarium specimens and live plants and seeds from him over a period of 20 years. He also actively exchanged plants with nurseries and botanical gardens in various parts of the world. In 1865 he published an article on "Natal fibres" in Technologist (Vol. 5, pp. 25-27). Two years later he visited Mauritius, particularly the excellent botanical gardens at Pamplemousses, and brought back some plants for the Durban Botanic Gardens. Though he was a good plant collector and botanist, he failed to label most of the plants he introduced to the botanic gardens.
McKen was particularly interested in ferns and in 1869 published a booklet, The ferns of Natal, based on W.J. Hooker's Synopsis Filicum. It listed the locations where plants had been collected by himself, Gerrard, Reverend John Buchanan*, and John Sanderson*. The contents were also published in the Natal almanac and yearly register for 1870 (pp. 33-60). With Gerrard as co-author he furthermore published the booklet Synopsis Filicum Capensium (Pietermaritzburg, 1870).
In August 1868 McKen addressed the recently established Natural History Association of Natal "On the economic advantages of studying natural history". He undertook to collect lepidoptera in Natal for the South African Museum in 1865. Many years later Roland Trimen* of the South African Museum, in the preface to his book South African buterflies... (1887-1889) thanked McKen for a large collection of butterflies collected at Durban. Trimen also named one of his discoveries, the butterfly Pamphila mackenii, in his honour. McKen also collected many birds, presumably to sell them. These are now in the museum of the University of Cambridge, but include no new species.
From 1867 to his death McKen served on the committee of the Durban Horticultural Society and from 1870 also on the council of the Natal Chamber of Agriculture. He was short and stockily built, with a confident and lively disposition, and was known informally as "the professor". However, he had suffered from malaria since visiting Mauritius and was a heavy drinker, and when he died at the age of 49 he left his wife and six children destitute. After Gerrard he was the most active of Natal's early plant collectors and his specimens were cited throughout the Flora Capensis. He also took the trouble to record the Zulu names of various plant species. He forwarded plants to Kew from other collectors such as Edward Button* and John Sanderson, though the latter accused him of various misdemeanours during the last years of his life. A memorandum book with a record of his activities is preserved in the Natal Herbarium. The genus Mackenia was named after him by Harvey while he was commemorated also in the names of the orchid Eulophidium mackenii, the lilly Eriospermum mackenii, the gourd Peponia mackenii, and several other species. Specimens from him are housed in the National Herbarium at the Botanical Research Institute in Durban, and the herbarium of the Royal Botanical Gardens, Kew.