Conrad Martens was born in London in 1801, the son of J. C. H. Martens, who had settled in England after serving as the Austrian consul. In the early 1820s, Conrad Martens studied landscape painting under watercolourist Copley Fielding (1789--1855), and for the next ten years undertook a number of sketching tours, mainly in Kent and Devon; two Devon scenes were exhibited in 1833 at the Royal Society of British Artists.
In 1833 he sailed on the Hyacinth from Plymouth via Gibraltar, Funchal, Tenerife and Salvador to Rio de Janeiro, where he made numerous pencil sketches. The Hyacinth was bound for India, but while in Rio, Martens learned of a vacany on the British South American Expedition ship Beagle, and sailed on the Indus to Montevideo in July 1833 to join the crew in place of the artist Augustus Earle, who had become too unwell to continue.
The Beagle, accompanied by the rescue ship Adventure, finally left Montevideo on 6 December 1833, reaching Port Desire on the Patagonian coast by Christmas. In the first half of 1834 the Beagle visited Port St Julian, Port Famine, Tierra del Fuego, and Port Louis on East Falkland Island. In Tierra del Fuego, Martens drew Jemmy Button, a young Fuegian educated in England by FitzRoy, and of interest to the Beagle naturalist, Charles Darwin, in the context of differences between `primitive' and `civilised' human peoples.
After a stormy passage through the Straits of Magellan, the Beagle visited the island of Chiloé and reached Valparaiso in Chile on 31 July 1834. The British Admiralty thereafter refused to underwrite the expenses of the rescue ship, which FitzRoy consequently sold, one upshot being that there was no longer room on the expedition for Martens. He stayed with a German named Berger in Chile, and undertook a painting tour with J. M. Rugendas, sailing on the Peruvian in December 1834 to Tahiti and Moorea, where he stayed until 4 March 1835.
Martens sailed from Tahiti for New Zealand on the Black Warrior, and continued to Sydney in Australia, arriving there on 17 April 1835, armed with a letter of introduction from FitzRoy to Captain Philip Parker King, former commander of the British South American Survey. King and his cousins became friends and patrons to Martens, who sketched in the Sydney, Blue Mountains and Illawarra regions, and began to receive commissions.
1836 was financially the most successful year of Martens' life as an artist. The Beagle arrived in Australia that year, and Darwin visited Martens, commissioning watercolours of Jemmy Button waving farewell to the Beagle, and of an excursion up the Santa Cruz River. FitzRoy commissioned a view of Moorea, and was presented with a view of Tahiti. Other large commissions were also received, and the following year some of Martens' Australian watercolours were exhibited at the Royal Society of British Artists in London.
Martens married Jane Carter on 9 March 1837, and had two daughters, Rebecca (1838--1870) and Elizabeth (1839--1909). A son, William, was born in 1844 but died aged only six weeks. In 1839 drought in Australia was followed by an economic recession lasting until the late 1850s. Commissions became scarce and the Martens family experienced an extended period of financial difficulty. Martens made some money by selling prints from a lithograph of `Sydney from the north shore', and by taking pupils. He exhibited watercolours at the inauguration of the Society for the Promotion of the Fine Arts in Australia in 1847, and in its subsequent exhibitions.
A period of instruction in oil painting from Marshall Claxton in 1850 lightened Martens' formerly `muddy' palette and improved his technique. He toured parts of eastern Australia in the early 1850s in the hope of finding new patrons, visiting Philip Gidley King in 1852 and receiving news of Darwin. He exhibited at the Victorian Fine Arts Society in Melbourne in 1853, and at the Paris Universal Exhibition in 1855. Eventual improvement in the Australian economy in the later 1850s led to an increase in significant commissions.
In 1862 Martens received a message from Darwin, and replied, congratulating him on the success of the Origin of species. He sent Darwin a watercolour of Brisbane River on the occasion of a visit to Darwin in October 1862 by Wickham, former Government Resident in Brisbane, and other former Beagle shipmates. Paintings by Martens were exhibited at the International Exhibition in London.
In 1863 Martens became Assistant Librarian in the Australian Parliamentary Library, securing his financial position, but severely curtailing the time he could spend on artistic work. He exhibited at the Paris International Exhibition in 1867. He received his first public commission in 1872, from the Victorian Gallery (later National Gallery of Victoria) for a watercolour of Apsley Falls near Waterloo, and a second similar commission in 1875 from the New South Wales Academy of Arts (later Art Gallery of New South Wales), of whose Council he became a member in 1877.
From the later 1860s Martens suffered from angina, and he died from a heart attack on 21 August 1878.