John Lessels was born in Kirkcaldy on 9 January 1809 (his gravestone wrongly gives his date of birth as 1808) and was the son of John Lessels (1784-1865) clerk of works on the Raith Estate, and his wife Elizabeth Hamilton Murray. His grandfather, also John Lessels, had been not only an architect-builder but a mill-wright and these practical skills were passed down the family. He was educated in Kirkcaldy, being taught by the Reverend Edward Irving and Thomas Carlyle. After leaving school he first worked as a carpenter for his father, but sought leave to become an architect. This proposal was discouraged but at the suggestion of his father and Robert Fergusson of Raith he entered the office of William Burn, achieving the position of inspector of works. He was in charge of Burn's work at Charleton, Fife in 1832. This enabled him to marry Mary, daughter of Robert Henderson, a Kirkcaldy bleacher, and from 1833 the Lessels family was in the Borders, first at Dawyck Peeblesshire where their eldest son John was born, and then at Allanton Berwickshire from which Lessels supervised the building of other Burn houses.
By 1843 Lessels had settled in Edinburgh with a house at 3 St Bernard Row and began exhibiting at the Royal Scottish Academy, initially purely as a watercolourist. In 1846 he set up practice on his own account in a flat at 7 St Vincent Street. His past experience with Burn in the Borders had made useful connections, and with Burn's departure for London in 1844 it became possible to seek clients there, the earliest being the Montgomerys of Stobo and Kinross. By the mid-1850s he had also secured the business of the Walker Estate which gave him architectural control over a large part of the Western New Town, and in 1856 or 1857 he formed a partnership with his son John who was in charge of a branch office at 19 Princes Street in Perth, but this closed c.1859 as a result of a local fee-cutting war. By the early 1860s, Lessels had gained the patronage of Nelsons the publishers, and his friendship with the city architect David Cousin, then in failing health, resulted in his appointment as joint architect to the City Improvement Trust on 23 May 1866, their first proposals for St Mary Street being presented in June.
Lessels's improved circumstances were reflected in his move from St Vincent Street to 21 Heriot Row which was both house and office, but in that year his first wife died, having raised four sons and four daughters. He would appear to have gone into semi-retirement at the age of seventy in 1879 when the office was moved to 50 George Street. His eldest son John having entered the service of the Crown as Surveyor of Windsor in 1860, the practice was now carried on by his second son James.
Lessels's first pupil was David MacGibbon who wrote of him that 'his ability, together with his perfect honesty and fearless impartiality, had gradually raised him, in spite of many adverse circumstances, to a high position among his fellow citizens… his modesty prevented his full merits as an architect from being recognised and appreciated'. Of Lessels's other pupils the most important were Robert Rowand Anderson and Robert Lawson who emigrated to Australia, then moved to New Zealand.
By the time of Lessels's retirement he had married a second time to Gertrude Anna Huberdina Nofken (anglicised Naufkins) of Baardwyk, North Brabant, and was spending much of his time on the continent, particularly in Belgium, Salzburg and Venice. These continental connections seem to have commenced earlier and strongly influenced his competition designs. His German Gothic design submitted in the limited competition for St Mary's Cathedral in 1873 was placed last by the assessor Ewan Christian whose report was published. This may have discredited him with the Walker Trustees, but the loss of the Walker Estate connection had begun slightly earlier in 1872 because Peddie and Kinnear and companies controlled by them were the largest feuars.
Lessels's acquaintance with Venice similarly influenced the Venetian Gothic design he and David Cousin submitted in the Edinburgh New Buildings competition of 1874 but this was no more successful, Rowand Anderson's North Italian Renaissance design being selected. John Lessels died of cancer of the liver on 12 November 1883 and was buried in Dean Cemetery, commemorated by a very curious sundial monument which formerly bore a photographic glazed tile portrait. It also bears the tools of the mason trade, probably a reference to his Masonic interests: he was a member of the Old Kilwinning Lodge. His second wife survived him by only a few months, dying at Duffel, Belgium on 12 May 1884. There were no children of the second marriage.
Of Lessels's personal life we know a little from his son John's memoir: 'In his youth he was a good athlete, and like his father a good marksman with guns made by himself, he was also fond of both sea-fishing and fishing for trout and salmon, all his rods and tackle including flies being of his own make. He made telescopes, bound his own books and was a turner in both wood and metal, the lathe being made by himself.'
The sale catalogue of his effects (Dowell, 16 February 1884) shows that he owned two Kinnear cameras, a Meagher folding camera, a pantascopic and several other specialised cameras and numerous Dallmeyer, Steinheil and Voigtlander lenses, together with much other photographic equiptment. He was for several years President of the Edinburgh Photographic Society. He also had a significant picture collection, and frequently exhibited oil paintings of topographical and architectural subjects at the Royal Scottish Academy.