Marmaduke George Nixon was born probably in 1813 or 1814, in Valetta, Malta, the son of Henry Nixon, an army officer, and his wife, Elizabeth Browne. After graduating from the Royal Military College, Sandhurst, he was commissioned in 1831 as an ensign in the 39th (Dorsetshire) Regiment of Foot, which he accompanied to India. Nixon was promoted to lieutenant in 1833 and fought in the Coorg campaign of that year. Gazetted a brevet major in 1843, he took part in the Maharajpur conflict. In 1846 Nixon was gazetted a major, but with two unmarried sisters to support he found himself unable to bear the expenses of a field officer's lifestyle in India. He resigned his commission in 1851 and on the recommendation of his friend Theodore Minet Haultain, emigrated to New Zealand in 1852, aboard the Cresswell.
Nixon purchased a farm at Tautauroa, near Mangere, and soon became an advocate of the demands of South Auckland landholders for access to Maori 'waste land' in the Waikato and Thames regions. Soon after the outbreak of war in Taranaki Nixon approached the government offering to raise colonial volunteers. Gazetted a lieutenant colonel in the Auckland Militia and the Royal Volunteer Cavalry in 1860, he was thereafter responsible for guarding the line of communications and supply between Auckland and the South Auckland redoubts and outposts. His force was partly responsible for the defence of Otahuhu, Panmure and Howick, and he organised flying columns against marauding war parties. In May 1863 the Colonial Defence Force Cavalry was formed, with two troops from the Auckland district; Nixon became its commandant in June. At Otahuhu in July 1863 Nixon recruited nearly 200 volunteers from the young farmers of the area – the nucleus of 'Nixon's Horse'.
Nixon's cavalry moved into Waikato with Lieutenant General Duncan Cameron's invasion force in July 1863. In February 1864 Cameron bypassed the Kingite fortress at Paterangi and ordered an attack on the Maori supply base at Rangiaowhia, a village just over four miles from present day Te Awamutu. The attack of 21 February was unexpected by the village's defenders, but a small garrison of aged warriors offered a fierce resistance. Nixon's colonial cavalry was in the vanguard of the attack and dismounted to pour concentrated fire on a single building where the last defenders had gathered. Nixon led an assault on this position and was shot at the entrance. He received severe chest wounds and died on 27 May. Nixon's troops reacted to his shooting by killing Maori who attempted to surrender or escape the building, which was either set on fire deliberately or ignited by sparks from the heavy gunfire. The troops lost five men, either killed outright or mortally wounded, and the Maori lost around 12 men and women. Twelve prisoners were taken and twenty-one women and children were detained.
Nixon was a member of the House of Representatives at the time of his death, having being elected member for Franklin in 1861. A public outburst against those responsible for his death followed and on 30 October 1865 the New Zealand legislature passed an act to grant a pension of £150 per annum to his surviving sisters, Catherine Elizabeth Nixon and Anna Susannah Nixon. He was buried in Auckland's Grafton cemetery and a monument in his honour was erected at the junction of the Great South and Mangere roads.