Hals was born at Tresawen, Merther, Cornwall, the second son of James Hals of Fentongollan and Anne, daughter and coheir of John Martin of Hurston, Devon. His father, a younger son of Sir Nicholas Halse (d. 1636), served at La Rochelle in 1628, and afterwards in the West Indies, where, according to his son, he was governor of Montserrat; he held Tresawen by lease from his mother.
Nothing is known of Hals's education. Although his publishers claimed he was a ‘perfect master of the Cornish and very well vers'd in the British and Saxon, as well as the Learned Languages’ (Hals, printed wrapper), his etymology is poor. His surviving manuscripts include a translation of John Keigwin's ‘Mount Calvary’ (BL, Add. MS 28554, fols. 51–8). He began researching the history of Cornwall about 1685 and pursued his interest for the rest of his life. The manuscript of his ‘Parochial history of Cornwall’ (BL, Add. MS 29762) has the appearance of a working copy and, although described as nearly completed at his death, it seems unlikely that he would ever have published the work. According to his own account in his history, his first two wives belonged respectively to the families of Evans of Llandrinio in Wales and Carveth of Perran sands; nothing else is known of them. In 1714 he married Jane Courtenay of Tremere (b. 1672); they had no children, and his wife died some time before 1736. Hals died, probably in 1737, at Tregury, St Wenn, of which he owned the rectorial tithes.
Hals's manuscripts passed to William Halse (d. 1775) of Truro, who about 1750 arranged for the Compleat History of Cornwall to be published by Andrew Brice of Exeter in weekly sixpenny numbers of four sheets. This appears to have been a financial rather than a scholarly venture. The publishers began with the second part of the work, a parochial history taken directly from the manuscript, alleging that the introduction awaited ‘considerable additions … by a very great hand’. Hals's ‘History of St Michael's Mount’ and ‘Dictionary of the Cornish language’ were also intended as part of the final work. It seems that the venture was not a financial success and only seventy-two parishes (Advent to Helston) appeared. The suspension of the work was said to have been due to the scurrilous anecdotes it contained, although Lysons blamed the inaccuracies and ‘tedious’ legends of saints. Its scholarly apparatus was deficient by contemporary standards and it lacked the extensive genealogies and lavish illustrations of a work such as Dugdale's The History and Antiquities of the County of Warwick which might have encouraged the Cornish gentry to subscribe. In style the work resembles the county histories of the early seventeenth century, in which Cornwall was amply represented by Richard Carew's Survey of Cornwall. It is likely that the majority of gentry families already possessed a copy of Carew or of John Norden's Topographical and Historical Description of Cornwall (1728), and felt little inclination to subscribe to the new work. Hals's manuscript was, however, incorporated into the nineteenth-century parochial histories of Cornwall produced by Davies Gilbert and Joshua Polsue.