Joseph Zillwood was baptised at Cholderton, Wiltshire, England, on 23 December 1804. He was the son of Thomas Zillwood, a farmer, and his wife, Elizabeth Silver. Little is known of his early life, although he worked as an agricultural labourer and at Laverstock Asylum, near Salisbury. On 26 October 1836, already a widower, he married Betsy Rose, at Salisbury. The couple soon moved to France, where Joseph worked as a coffee-house keeper in Le Havre, and where their children Elizabeth Ann and Thomas were born. In November 1839 the Zillwoods embarked for Port Nicholson (Wellington), New Zealand, on the Bolton, arriving on 21 April 1840; Elizabeth Ann died en route. Two more children were born in Wellington, Mary Ann in 1841 and William in 1844. In the latter year the family lived in Hawkestone Street, and Zillwood was employed as a labourer. In December 1845 he became a widower for the second time.
Zillwood served in the local militia in 1846, and later became a private in Wellington's Armed Police Force. In 1849 he was offered the position of chief constable at Akaroa, probably on the strength of his knowledge of the French language and his good conduct while in the Wellington police. The small settlement's two-man force was the only one in the colony not formally part of an armed police force; Zillwood and his constable were directly responsible to the town's resident magistrate, John Watson. This unique arrangement resulted from local circumstances: it was desirable to avoid antagonising, by an overt display of force, the French settlers who comprised the core of the Akaroa community, and neither the Maori nor the Pakeha population was large or troublesome.
Zillwood's promotion led him to put Mary Ann and William in care, and young Thomas out to work. But when he arrived at Akaroa after a storm-racked voyage, he discovered his salary was to fall by 3d. per day from the meagre 3s. 6d. of an armed police force private, itself lower than the prevailing rate for labourers. Not until May 1853 did Zillwood's salary return to its 1849 level. By this time the outflow of labourers to the Australian goldfields ensured that the lowest paid of those remaining could earn 5s. per day, and the demands of the new Canterbury settlement had further pushed up living costs, already high because of the area's remoteness. Zillwood could not keep up payments for the care of his younger children; eventually the New Munster government reprimanded him for this 'improper behaviour' and deducted the arrears from his pay.
His financial plight was not eased by his marriage to Ellen Catherine Keogh, a widow, who had arrived in Akaroa with her brother in April as part of the town's first organised influx of English settlers. According to family information this marriage took place at Akaroa on 22 June 1850. Nor did additional duties as Akaroa's postmaster sufficiently augment his salary. His policing tasks were taxing: police at this time had no days off, were typically on duty from 7 a.m. to 11 p.m., and also had charge of lock-ups, and of hard-labour gangs which frequently comprised dangerous criminals.
Mary Ann and William Zillwood were eventually reunited with their father, but William died in January 1853. In October Joseph Zillwood lost his job, without compensation, as a consequence of the transfer of policing to provincial government control. Akaroa's two-man force was halved, ostensibly because police were no longer required to carry the mails, but also because of the penury of the new administration and the declining importance of the peninsula relative to Christchurch and Lyttelton. Zillwood became a farmer and proprietor of an accommodation house at what is now called Duvauchelle, at the head of Akaroa Harbour.
As Zillwood's financial problems worsened, he turned to drink. During the winter of 1854 Ellen left him. A week after she had cut off all contact with him he shot himself in the mouth with his police pistol and, after suffering great pain for six days, died on 19 October 1854 at Akaroa. A coroner's jury duly found that he had 'feloniously voluntarily and of malice aforethought himself killed', and his body was 'interred at night, without funeral rites, pursuant to the Coroner's Warrant'. The property owners whose interests he had been employed to protect thus delivered a final, posthumous blow to Zillwood's dignity.