Samuel Horatio Moreton was born probably in London, England, sometime between 1843 and 1845, the son of Ann Spence and her husband, Samuel Moreton, said to have been a captain in the Royal Regiment of Artillery. Joining the Royal Navy, he served in China before emigrating to Australia. There, at Sandridge, Victoria, on 25 September 1862, he married Rosa Clara Wilson, the daughter of a London watchmaker.
Shortly after their marriage Rosa and Samuel Moreton moved to Invercargill, New Zealand, where Samuel worked as a painter and signwriter. During his early years in Invercargill he developed his talent for fine art painting: he is said to have studied earlier under the watercolour artist Aaron Penley of the Royal Military Academy, Woolwich. He also found delight in the mountains and lakes of Fiordland. These two interests Moreton combined for the rest of his life, becoming a landscape artist of international renown and a pioneer explorer of New Zealand's south.
Moreton made a major venture into Fiordland in 1881, and over the next 10 years he returned repeatedly for prolonged visits, often accompanied by W. Y. H. Hall, an Invercargill solicitor. He encountered Donald Sutherland, the first settler and at that time sole resident at Milford Sound, and came to know him well.
The spectacular beauty of Milford Sound was widely known before 1880 and its value as a scenic resort recognised. However, its inaccessibility prevented it from being exploited as a tourist destination. There was an urgent need to find a route suitable for tourists, and, spurred on by the financial rewards that finding such a route would provide, Moreton, Sutherland and others spent much time in the search. Moreton evidently believed that the Poseidon River which runs into Lake Ada near Milford Sound was the key to the route from Milford to Lake Te Anau, for he made at least three major expeditions into the area. The route to Milford Sound was finally discovered by Quintin McKinnon (Mackinnon) in October 1888, by what became known as the Milford Track. Moreton made two of the earliest crossings of the Mackinnon Pass after its discovery, on the first occasion accompanied by his wife, Rosa, who would thus appear to have been the first woman to walk the Milford Track.
Samuel Moreton's contribution to the opening up of Fiordland included a solo trip from Milford Sound to Queenstown via Martins Bay, and pioneering a route to Bligh Sound. His art was probably the greatest single factor in advertising the scenic beauty of the area and the uniqueness of Milford Sound. His ability to capture the grandeur of Fiordland soon became well known, and his work was in demand both in New Zealand and overseas. He was commissioned to do a series of paintings for the Illustrated Sydney News in 1887, and later for the Illustrated London News. Five of his works were exhibited at the Colonial and Indian Exhibition in London in 1886. In New Zealand he exhibited with the art societies in Dunedin, Auckland and Christchurch, and at the New Zealand Industrial Exhibition in Wellington in 1885.
In 1890 or 1891, in debt and his marriage having failed, Moreton moved to Christchurch, and there established an art school, the Pre-Raphaelite School of Art. He regularly took his students on sketching tours to scenic areas, often to Fiordland. Rosa Moreton remained in Invercargill, where she died on 10 May 1908, survived by seven children. The following year, at Christchurch on 8 November, Samuel Moreton married Amelia Rose Woolley, an artist he had previously taught; there were no children of this marriage. Amelia Moreton published a volume of poems and a descriptive sketch of their travels in the upper Rakaia area.
Samuel Moreton died in Christchurch on 21 March 1921, of cancer. Most of his works are now in private hands, but a number are held by the Alexander Turnbull Library, Wellington, and the Robert McDougall Art Gallery, Christchurch.