Josiah Child was born c.1630 (baptised 1631), the second son of Richard Child a merchant of Fleet Street (buried 1639 at Hackney) and Elizabeth Roycroft of Weston Wick, Shropshire. Although the Oxford Dictionary of National Biography states positively that he was not related to the Child & Co bankers of Osterley Park, Burke's Armorials 1884 provides evidence to the contrary, giving both families the same armourials: "Gules, a chevron ermine between 3 eagles close argent". (See Villiers, Earls of Jersey, into which family the banking Child family married). The earliest bearer of these Child arms was William Childe, sheriff of Worcestershire in 1585.

Josiah married firstly, Hannah Boate, dau. of Edward Boate, on 26 December 1654 at Portsmouth, Hampshire, by whom he had one surviving child, Elizabeth, 2 others having died young. He married secondly, c. 14 June 1663, Mary Atwood, daughter of William Atwood, by whom he had Rebecca (c.1666 - 17 Jul 1712) and his heir Josiah Child, 2nd Baronet (c.1668 - 20 Jan 1704). He married thirdly, c. 8 August 1676, Emma Barnard daughter of Sir Henry Barnard, by whom he had Richard Child (5 Feb 1680 - March 1750), who was created Viscount Castlemain in 1718 and Earl Tylney in 1731. He died on 22 June 1699, and was buried at Wanstead, Essex. His will dated 22 February 1696 was proved on 6 July 1699.

After serving his apprenticeship in the family business, to which after much struggle he succeeded, he started on his own account at Portsmouth, as victualler to the navy under the Commonwealth, when about twenty - five; he is also described as "agent to the Navy Treasurer". He amassed a comfortable fortune, and became a considerable stock holder in the East India Company. He was returned to Parliament in 1659 for Petersfield; and in later years sat for Dartmouth (1673 – 1678) and for Ludlow (1685 – 1687). He was created Baronet Child of Wanstead in the County of Essex, in 1678. His advocacy, both by speech and by pen, under the pseudonym Philopatris, of the East India Company's claims to political power, as well as to its right of restricting competition to its trade, brought him to the notice of the shareholders, and he was appointed a Director in 1677, rising to Deputy - Governor and finally Governor. In this latter capacity he directed the Company's policy as if it were his own private business. He and Sir John Child, president of Surat and governor of Bombay (no relation according to the Oxford Dictionary of National Biography, arms: "Vert, 2 bars engrailled between 3 leopards' faces or") are sometimes credited with the change from unarmed to armed traffic, but the actual renunciation of the Roe doctrine of unarmed traffic by the Company was resolved upon in January 1686, under Governor Sir Joseph Ash, when Child was temporarily out of office.

He lost the war with Aurangzeb, 6th Mughul Emperor, between 1688 and 1690. Aurangzeb, however, did not take any punitive action against the company and restored its trading privileges. "For a massive indemnity and promises of better conduct in the future, he [Aurangzeb] graciously agreed to the restoration of their [East India Trading Company's] trading privileges and the withdrawal of his troops".

Child made several important contributions to the literature of economics; especially Brief Observations concerning Trade and the Interest of Money (1668), and A New Discourse of Trade (1668 and 1690). He was a moderate in those days of the mercantile system, and has sometimes been regarded as a sort of pioneer in the development of the free trade doctrines of the 18th century. Though Child considered himself a proponent of the competitive market, he simultaneously argued for a government controlled interest rate and restricted trade among the colonies which would benefit England. He made various proposals for improving British trade by following Dutch example, and advocated a low rate of interest as the causa causans of all the other causes of the riches of the Dutch people. This low rate of interest he thought should be created and maintained by public authority. Child, whilst adhering to the doctrine of the balance of trade, observed that a people cannot always sell to foreigners without ever buying from them, and denied that the export of the precious metals was necessarily detrimental. He had the mercantilist partiality for a numerous population, and became prominent with a new scheme for the relief and employment of the poor; it is noteworthy also that he advocated the reservation by the mother country of the sole right of trade with her colonies.

Child purchased Wanstead House in Essex in 1673 from the executors of Sir Robert Brooke and spent much money on laying out the grounds. The diarist John Evelyn made the following characteristically waspish entry for 16 March 1683

"I went to see Sir Josiah Child's prodigious cost in planting of walnut trees about his seat and making fishponds many miles in circuit in Epping Forest in a barren spot as commonly these over - grown and suddenly monied men for the most part seat themselves. He from an ordinary merchant's apprentice & management of the East India Company's common stock being arrived to an estate ('tis said) of £200,000 and lately married his daughter to the eldest son of the Duke of Beaufort, late Marquis of Worcester, with £30,000 (some versions £50,000) portion at present, & various expectations. This merchant most sordidly avaricious etc."

According to Daniel Defoe Child "added innumerable rows of trees, avenues and vistas to the house, all leading up to the place where the old house stood, as to a centre".

He served as High Sheriff of Essex in 1689.