Lachlan MacLean entered the service of Sir Donald Currie, founder of the Castle Steamship Company, on leaving school. At first he worked in the Glasgow office, but was later transferred to Leith and then to London. In 1878, six years after the company had started a line of steamers to Cape Town, MacLean was sent to the Cape. He remained with the company until it amalgamated with the Union Steamship Company to form the Union-Castle Mail Steamship Company in 1900, when he became its chief agent for South Africa. He resided in Kenilworth, Cape Town.
MacLean's hobbies included shooting and fishing. In 1883 he visited the Fisheries Exhibition in London and the next year imported 20 000 brown trout ova, largely at his own expense. Earlier attempts by others to hatch imported trout ova had all failed. Some 17 000 of MacLean's ova were hatched at the Waverley Mills wool-washery near Wolseley, but most died, probably as a result of zinc poisoning caused by the use of unvarnished zinc sheeting to line the troughs. The remaining small number of fry survived in a rearing pond but were later washed away by a flood. Only three fish were rescued and survived until 1890. Though the experiment was a failure, it showed that the introduction of trout was possible.
During 1890 imported trout ova were first successfully hatched in Natal by J.C. Parker, and established in the Bushmans and Umgeni Rivers. At this time MacLean was active in organising the government-sponsored introduction of brown trout ova to the Cape Colony. The first batch of 100 000 eggs arrived in March 1892 in the care of an English fish culturist, Ernest Latour*. The venture was managed by the Cape Town Fisheries Committee, headed by MacLean. A hatchery had been established at an old brewery in Newlands, where Latour supervised the hatching and rearing of the fry. Thus trout was successfully introduced in the Western Cape.
Around 1892 MacLean, with Roland Trimen* and others, served on a committee to investigate the expansion of the sea-fisheries industry. Their recommendations convinced the government of the Cape Colony to add a marine biological section to its Department of Agriculture, appoint marine biologist J.D.F. Gilchrist*, and acquire a fishing trawler, the Pieter Faure, with which to explore new fishing grounds.
In December 1886 MacLean was one of the founding members of the Western Districts Game Protection Association and was elected treasurer. In August 1891 he became also its honorary secretary, an office which he held continuously to his death in 1914. His correspondence on behalf of the association reflects his continued role in fresh-water fishery matters at the Cape. For example, in 1893, at the request of W.W. Thompson* of the Department of Agriculture, he wrote the first draft of Proclamation No. 56, promulgated on 12 February 1894, to provide legal protection to trout in Cape rivers. In September 1902 the association expanded its objectives to include the protection of all fresh-water fish, in particular trout, and on this matter MacLean became its principal spokesman. He collaborated with Gilchrist and Thompson in supervising the Jonkershoek hatchery and the policy to be followed in the distribution of trout fry, and participated in drawing up trout fishing regulations.
In April 1909 the Fishery Advisory Board was inaugurated in Cape Town, with MacLean as one of its members. At some time he was president of the Western Province Angling Association, founded in November 1904. He was furthermore a member of the Cape Town Harbour Board, a committee member of the Chamber of Commerce, and for five years served as mayor and municipal councillor of Claremont. In 1905 he became a member of the British Association for the Advancement of Science. That same year he also joined the South African Philosophical Society and remained a member when it became the Royal Society of South Africa in 1908. By January 1914 he was seriously ill and died before the beginning of April. He was praised for his zeal and unremitting labour in the preservation of game and trout and his invaluable services to the Game and Trout Protection Association.