According to family information Robert Maunsell was born at Milford, near Limerick, Ireland, on 24 October 1810. He was the seventh child of George Maunsell, a collector of customs and later a banker, and his second wife, Frances Magrath Fitzgerald. Robert Maunsell received his early education at Waterford, before entering Trinity College, Dublin, where he graduated in 1833 with honours in Classics, having won a prize in Hebrew. He then attended the Church Missionary Society Training College at Islington, London, England, where his teachers included the scholar of Māori, Samuel Lee. He was ordained deacon in December 1833 and priest in December 1834. Shortly after his ordination, in 1834 or 1835, Robert Maunsell married Susan Cherry Pigott, of Camberwell. There were four sons and three daughters of this marriage.
Robert and Susan Maunsell embarked for Australia on the Florentia in 1835. They continued to New Zealand on the Active, arriving at Paihia, Bay of Islands, on 26 November. In August 1836 they established, with James and Elizabeth Hamlin, a mission station on the southern side of the Manukau Harbour. In June 1839 the Maunsells moved to Maraetai at the Waikato Heads, a location chosen for its large number of Māori visitors. Until 1842 they were supported by Harriet and Benjamin Yates Ashwell. Within a year there were some 700 pupils in various classes around the district. Few Māori were unbaptised by 1843.
For six years Robert Maunsell conducted services over an area extending from Coromandel to Te Awamutu and to the Manukau Harbour. Initially he was the only Anglican clergyman in this area. He was also the first minister to conduct a Christian service in Auckland, in 1840. At Maraetai Maunsell gained 32 Māori signatures to an English text of the Treaty of Waitangi, although he failed to persuade Pōtatau Te Wherowhero of Waikato to sign. He believed the treaty would protect the Māori from the harmful effects of European settlement, and later complained that its guarantees were being undermined. In 1845–46 Maunsell mediated in a series of disputes involving Ngāti Pou, Ngāti Tipa and Ngāti Tamaoho. He travelled frequently, preaching, teaching and supporting the work of his lay colleagues, John Morgan at Te Awamutu and Benjamin Ashwell, now at Taupiri. During Robert Maunsell's absences Susan Maunsell, whose health was poor, remained at Maraetai. Until 1847 she was assisted by Mary Rymill in caring for the Maunsells' young family and teaching Māori girls.
Robert Maunsell regarded schools as 'the pivot and springs of mission success', and by 1846 he enthusiastically supported Bishop G. A. Selwyn's self-supporting 'industrial' boarding schools, where pupils lived in European style, taking this influence back to their villages. The Maunsells established such a school at Maraetai in 1847. At its height there were over 100 pupils at the school, one of the largest of its kind. It was regarded as a model, regularly earning praise from inspectors and distinguished visitors. Assistants at the school included some long-serving Māori teachers, most notably Kaitupeka (Mary) Ngataru, and Carl Völkner and James Stack. Stack found him a very hard taskmaster.
Susan Maunsell took charge of the girls' school as far as her health allowed. From 1851 she was assisted by Beatrice Isabella Duncan Panton. Born at Cupar, Fife, Scotland, on 26 November 1825, the daughter of Agnes Hoyes and her husband, George Panton, Beatrice Panton had arrived in Auckland in 1849 with her brother, a Presbyterian minister. When Susan Maunsell died in October 1851, Beatrice Panton took charge of the girls' school and of the younger Maunsell children. On 30 September 1852, at Auckland, Robert Maunsell and Beatrice Panton were married. They had two daughters and one son. A capable, courageous woman, Beatrice was well able to take charge of the large establishment in Robert's absence. She maintained the mission house and taught at the school for 13 years, as well as making visits around the district, teaching the women and tending the sick. She was admired for her conscientious teaching, although James Stack found her a formidable character: 'a Scotch puritan who rejoiced in bearing testimony, however ill-timed'.
In 1853–54 Beatrice and Robert Maunsell ran two establishments while a new site for the school was being prepared at Kōhanga, nine miles upstream. Waata Kūkūtai, chief of Ngāti Tipa, had donated 750 acres at Kōhanga, there being insufficient fertile land at Maraetai for the school farm. The school continued at Kōhanga until the mid 1860s, when the war in Waikato forced the Maunsells to abandon the station.
Robert Maunsell had described the Taranaki war of 1860–61 'as wicked & unjust a war as any that can be found in the pages of colonial history'. In 1860 he successfully intervened in a potentially explosive situation at Patumāhoe, and was later consulted by members of the King movement. The government, he felt, should acknowledge the Māori King. However, in 1863 he was convinced of the imminence of a Māori attack on Auckland, and, although he denounced the subsequent land confiscations, considered the invasion of Waikato in July 1863 to be justified. Despite the hostilities, Robert and Beatrice Maunsell remained in the area until evacuated in October 1863. They then removed to Auckland, where Beatrice remained with the family. Robert returned to Waikato as a chaplain to the government troops, trying to minister to both sides.
The Maunsells returned to Kōhanga as soon as possible to resume teaching and preaching. Shortly after, however, in October 1864, Beatrice Maunsell died at Kōhanga. Robert Maunsell now moved to the parish of St Mary's in Parnell, Auckland, after moves to secure him the bishopric of Nelson failed. He remained at Parnell until his retirement in 1882. He continued to be involved in the local CMS committee, and in diocesan administration as archdeacon of Waitematā in 1868 and archdeacon of Auckland from 1870 to 1883.
In these years Maunsell also continued to work on revisions of a Māori translation of the Bible, work which he had begun in the 1830s. An exceptionally able linguist, he had never missed an opportunity to discuss idioms with learned Māori and to record usages that were new to him. In 1842 he published Grammar of the New Zealand language. The following year, in a fire which destroyed the Maunsells' house, he lost the manuscripts of a dictionary and translations of the Old Testament. The dictionary was never restarted, but he resumed work on his translation of the Old Testament from Hebrew into Māori. Several books were published in the 1840s, and the project was completed in 1857. In addition he helped to revise the whole Bible for editions in 1868 and 1887, and also the Prayer Book. He was awarded an honorary doctorate by Trinity College, Dublin, for his work.
Robert Maunsell's intellectual ability won him the respect of men such as William Martin, George Grey, William Williams and George Selwyn, who consulted him about Māori language and culture, education, and church administration. He had a dry sense of humour, and was described by Sarah Selwyn as 'a racy Irishman, a very able man and with a rare knowledge of Māori'. His evangelical views and staunch independence, however, often brought him into conflict with Bishop Selwyn, for he was forthright in expressing his opinions and quick to take offence. In particular, he criticised Selwyn's reluctance to ordain Māori, firmly believing in the need for Māori leadership of the Māori church. Although Maunsell believed that 'communalism' was the greatest obstacle to Māori progress, his exceptional knowledge of the language made him more sensitive to Māori culture than most Europeans. He respected and was respected by many Māori. He died on 19 April 1894 at Parnell.