Allan McLean was baptised on 24 May 1822, the seventh of nine children of Alexander McLean and his wife, Mary McLean, of Lagmhor on the Inner Hebridean island of Coll, Scotland. The Isle of Coll had been inhabited by the McLean clan since the fifteenth century.
After the death of Alexander McLean, a farmer and fisherman, in 1836, and subsequent widespread crop failure, Mary McLean took her three surviving sons, John, Allan and Robertson, and two daughters, Alexandrina and Mary, to Australia in 1840. The brothers found employment as shepherds, and then bought two west Victoria runs, which they owned between 1848 and 1851. During the Victoria goldrush they worked as carriers, merchants and gold buyers at Ballarat and Bendigo. In 1852 John, Allan and Robertson McLean took up the Ashfield run, near Christchurch, and the family shifted to New Zealand.
The McLeans' runholding expanded rapidly in the 1850s. They bought the Acheron Bank run in 1853 and sold it a year later, and then in 1855 took up the large Lagmhor run, near Ashburton. Robertson McLean was the manager of this run at first, but he soon returned to Scotland and died there in 1871. The brothers extended their runholding to Otago in 1858 by taking up the Morven Hills run of between 400,000 and 500,000 acres, which was named after the Scottish mainland near Coll. At the time this was the largest run in New Zealand. They also acquired the Waitaki Plains and Redcastle runs.
In 1866 John and Allan bought the mainly lowland Waikakahi run, near Waimate, and took their sister Alexandrina's husband, George Buckley, into partnership. They obtained the freehold of most of the run. John and Allan bought out Buckley's share in 1875, and then in 1880 dissolved their partnership. Allan chose to retain Waikakahi, which became one of the finest stations in Canterbury. It was renowned for its excellent cropping and stock. The flock rose to 69,000 sheep in 1895, and at one time up to 40 four-horse teams could be seen ploughing in one block of 8,000 acres. The 21-roomed homestead, known as The Valley, was surrounded by magnificent gardens. Here Allan McLean entertained in a lavish fashion. He did not marry, but his home was capably managed by a housekeeper, Mrs Emily Phillips.
Allan McLean was a somewhat eccentric figure. He frequently wore a plum-coloured suit, bow-tie and white socks, and travelled around the district in a white wagonette referred to locally as 'The Yankee Express'. However, he was also known for his generosity to the poor and had a large bunk-room for swaggers built on his property.
By the end of the century the Liberal government was encouraging the break-up of large freehold stations, by compulsory purchase if necessary, under the Land for Settlements Act 1894. The Waikakahi run of some 48,000 acres was bought for about £320,000 in 1899. There was intense demand for the 130 farms, 14 runs, and 47 village sections in this, the second-largest government settlement in Canterbury. Allan McLean was distressed at having to part with Waikakahi, and never returned to the district.
Retiring to Christchurch, he commissioned Robert West England to design a Jacobean-style, three-storeyed house of 53 rooms. Construction was completed in 1900. The house was named Holly Lea: holly was the McLean plant badge. At 23,000 square feet, it was believed to be the largest wooden residence in New Zealand at that time.
Allan McLean died at Holly Lea on 12 November 1907. Bequests were left to his relatives and servants, including Emily Phillips, who was also given the right to live in Holly Lea for the rest of her life. When she vacated the house in 1913 it came under the control of the McLean Institute, which was established by the terms of McLean's will for the purpose of providing 'a home for women of refinement and education in reduced or straitened circumstances'. McLeans Mansion, as the building became known, was used for this purpose until 1955.