John Lessels was born on 22 December 1833 at Dawyck Peeblesshire where his father John Lessels Senior was William Burn's inspector of works. He was the eldest son. From Dawyck the Lessels family moved to Allanton to supervise the building of other Burn country houses and there John Junior first went to school. Thereafter his father sent him to his old school at Kirkcaldy, probably because he was then supervising Burn projects in Fife, and finally to the Edinburgh Institution where John Junior was dux of all his classes in 1846-48. He was then apprenticed to his father who was now in independent practice. He studied at the Trustees' Academy from 2 April to 12 July 1851 on the recommendation of Charles MacGibbon, whose son David had been an apprentice in the Lessels office, and took classes in clay modelling from Gourlay Steel RSA and in watercolour from John Winton, ARSA. From 1853 onwards he sent to the RSA, his exhibits being listed with his father's in RSA Exhibitors 1826-90 as he was living at the same address.
In September 1853 Lessels was appointed Drawing Clerk at Windsor Castle on the strength of a testimonial from Professor Lees. His duties were 'to make plans, elevations and models in accordance with the ideas of HRH Prince Albert for the improvement of the castle both externally and internally and to attend during theatrical performances in charge of The Fire Brigade'. This post terminated in 1856 when the funds allocated by the Treasury were exhausted, but while at Windsor he taught at the Mechanic's Institute and married Margaret Gillies, his eldest son John being born on 13 September 1856.
In 1856 or early 1857 Lessels returned to Scotland as his father's partner to open a branch office in Perth. This prospered for two years but eventually closed as a result of fee cutting - 2 1/2% against John Lessels and Son's 5% - and a distaste for the drinking habits of the local builders and their suggestion of fraud on clients to make up his fees. While in Perth his infant son John died in 1857 and his second son, also John, was born on 18 April 1858.
On his return to Edinburgh Lessels set up practice as 'a general designer and lithographer' specialising in furniture, stained glass and book stamps, but although he described that practice as 'lucrative' he decided to emigrate to New Zealand where he had relatives; his father's assistant Robert Arthur Lawson, who was exactly the same age, had already emigrated to Australia.
For that purpose he spent part of each day as a carpenter to be sure of employment. He then proceeded to London to learn English methods of carpentry with George Myers, who had been Pugin's builder. He was sent to one of their branch workshops at Windsor where he was entrusted with the execution of the Gothic doors he had designed in 1855. As a result of his return to Windsor he was offered a further appointment with the Office of Works, but at first declined as arrangements had been made for his passage to New Zealand. In the event his friends wrote to pay that because of the bankrupt state of the colony the only employment they could find for him was as a shepherd for the Bishop's sheep. An increasing family induced him to sit the Civil Service Commissioners' and Office of Works examinations and he re-entered the service of the Crown in 1860, his earliest important task being the design and decoration of the large temporary building erected for the marriage of the Prince of Wales and Princess Alexandra in March 1863. Thereafter he attended to all works required for Royal weddings and funerals until his retirement and in his 'leisure hours' he had a private practice in architecture and illuminated testimonials. Like his father he made his own carved furniture and fishing rods, and although there is no mention of it in his memoirs, in 1860 he appears to have had a house at Marion Villa, Foresthill Kent (see RSA Catalogue 1866, no 456).
In 1870 Lessels was selected to rebuild the British Embassy in Constantinople, built by William James Smith with advice from Charles Barry in 1842-54. It was of fireproof construction but had been badly damaged in the Pera Fire of 1870. He set out to visit the site in October 1870 but because of the Franco-Prussian War he could not take the direct route which gave him the opportunity of visiting Antwerp, Mayence, Nuremberg, Innsbruck, Venice, Trieste, Corfu, Athens and Smyrna. On his arrival he was appointed surveyor not only of the embassy houses at Pera and Therapia, but of the consular buildings, hospital, prison, doctor's house, and British cemeteries at Scutari and elsewhere on the Bosphorus. In the course of this work he suffered an attack of Asiatic Cholera and sunstroke. He returned home in August 1871 to draw up the scheme but returned in early 1872 with Charles Rayson as clerk of works. The estimated cost was £38,000. The work comprised iron girders to make the building more fireproof, reinstatement, and a new glazed roof to the court. The northern extension was to house the servants' hall and kitchens. New boundary walls were to be built, with two new gates and gatehouses, and new stables as a result of post-fire widening. The work was completed in 1876 at a cost of £39,500.
In 1876 Lessels returned to Windsor damaged in health and without the promised fees for the Constantinople work, the First Commissioner taking the view that he was not bound by the decisions of his predecessor. Perhaps by way of making amends he was now promoted as Surveyor of the newly created Country District which included not only Windsor Castle and its Parks, but Frogmore House and Grounds, the Military Knight's House, Hampton Court Palace, Parks and Gardens, Bushy House, Upper Lodge and the other houses in Bushy Park, Longford River, White Lodge, Richmond Park, Pembroke Lodge, Thatched House, Kew Palace, Cambridge House and other buildings in the Royal Botanic Garden and pleasure grounds at Kew, and Broadmoor Criminal Asylum. In that capacity he rebuilt the front gatehouse of Hampton Court as now existing. Either during those years or earlier he researched the history of the Lessels family and appears to have at times adopted the surname of Lecelles which he believed to be the original spelling.
In 1884 Lessels suffered a breakdown in health as a result of stones in the bladder and recuperated by taking the waters at the Contrexeville Springs in the Vosges. He retired early at the age of sixty to Thistlewood, Guernsey, in September 1894 on a pension of £396. He deid in 1914.