His father was William Lardner, a solicitor in Dublin, who wished his son to follow the same calling. After some years of uncongenial desk work, Lardner entered Trinity College, Dublin in 1812, and obtained a B.A. in 1817 and an M.A. in 1819, winning many prizes. He married Cecilia Flood on 19 December 1815, but they separated in 1820 and were divorced in 1835. About the time of the separation, he began a relationship with a married woman, Anne Maria Darley Boursiquot, the wife of a Dublin wine merchant. It is believed that he fathered her son, Dion Boucicault, the actor and dramatist. Lardner provided him with financial support until 1840. Whilst in Dublin, Lardner began to write and lecture on scientific and mathematical matters, and to contribute articles for publication by the Irish Academy.

In 1828 Lardner was elected professor of natural philosophy and astronomy at University College, London, a position he held until he resigned his professorship in 1831.

Lardner showed himself to be a successful popularizer of science. He was the author of numerous mathematical and physical treatises on such subjects as algebraic geometry (1823), the differential and integral calculus (1825), the steam engine (1828), besides hand-books on various departments of natural philosophy (1854-1856); but it is as the editor of Lardner's Cabinet Cyclopaedia (1830-1844) that he is best remembered.

The Cabinet Cyclopaedia eventually comprised 133 volumes, and many of the ablest savants of the day contributed to it. Sir Walter Scott contributed a history of Scotland and Thomas Moore contributed a history of Ireland. Connop Thirlwall provided a history of Ancient Greece, whilst Robert Southey provided a section on naval history. Many eminent scientists contributed as well. Lardner himself was the author of the treatises on arithmetic, geometry, heat, hydrostatics and pneumatics, mechanics (in conjunction with Henry Kater) and electricity (in conjunction with C.V. Walker).

The Cabinet Library (12 vols., 1830-1832) and the Museum of Science and Art (12 vols., 1854-1856) were his other chief undertakings. A few original papers appear in the Royal Irish Academy's Transactions (1824), in the Royal Society's Proceedings (1831-1836) and in the Astronomical Society's Monthly Notices (1852-1853); and two Reports to the British Association on railway constants (1838, 1841) are from his pen.

In 1840 Lardner’s career received a major setback as a result of his involvement with Mary Spicer Heaviside, the wife of Captain Richard Heaviside, of the Dragoon Guards. Lardner ran off to Paris with Mrs Heaviside, pursued by her husband. When he caught up with them, Heaviside subjected Lardner to a flogging; he was unable to persuade his wife to return with him. Later that year he successfully sued Lardner for ‘criminal conversation’ (adultery) and received a judgment of £8,000. The Heavisides were divorced in 1841, and in 1846 Lardner was able to marry Mary Heaviside. The scandal caused by his affair with a married woman effectively ended his career in England, so Lardner and his wife remained in Paris until shortly before his death in 1859. He was able to maintain his career by lecturing in the United States between 1841 and 1844, which proved financially rewarding.

He died in Naples, Italy, and is buried in the Cimitero degli Inglesi there.

Lardner became involved in a number of ill-advised public disagreements with Isambard Kingdom Brunel regarding technical matters, in which he came off the worse.

While Brunel was building the broad-gauge Great Western Railway, Lardner carried out some experiments with the company’s flagship locomotive, North Star. He asserted that, whilst the engine was capable of hauling 82 tons at 33 m.p.h., it was only capable of hauling 16 tons at 41 m.p.h. He also recorded excessive fuel consumption at higher speeds. Lardner attributed this to the greater wind resistance of broad-gauge engines. Brunel and his assistant Daniel Gooch carried out their own experiments on the same locomotive and found that the only problem was that the blast pipe was too small. This was easily rectified and the North Star’s performance immediately improved. At the next meeting of the company’s directors, Brunel triumphantly dismissed Lardner’s evidence.

Lardner also criticised Brunel's design of the Box Tunnel on the Great Western Railway. The tunnel had a 1-in-100 gradient from the east end to the west end. Lardner asserted that if a train's brakes were to fail in the tunnel, it would accelerate to over 120 m.p.h., at which speed the passengers would suffocate. Brunel pointed out that Lardner’s calculations totally disregarded air-resistance and friction, a basic error.

When Brunel was proposing to build SS Great Western for the 3,500-mile transatlantic passage to New York, at a meeting of the British Association for the Advancement of Science, Lardner stated that:

As the project [was] of making the voyage directly from New York to Liverpool, it was perfectly chimerical, and they might as well talk of making the voyage from New York to the moon 2,080 miles is the longest run that a steamer could encounter — at the end of that distance she would require a relay of coals.

Again, Brunel was able to show that Lardner’s calculations were too simplistic. The principle that Brunel understood, which Lardner did not, was that the carrying capacity of a ship increases as the cube of its dimensions, whilst the water-resistance only increases as the square of its dimensions. This meant that large ships were more fuel efficient, and could carry sufficient coal for the long voyage across the Atlantic. Brunel was proved right when the Great Western steamed into New York harbour with 200 tons of coal to spare.

Lardner is mentioned in Karl Marx's 'Das Capital' and was well respected as an economist. He mixed with the rich and famous. He was involved in the founding of the University of London and the first person to hold the post of Professor of Natural Philosophy and Astronomy there. He was influential in publicising Charles Babbage's Difference engine.

Whilst lecturing in America Lardner was paid by Norris Brothers, the largest firm of locomotive builders, to investigate a fatal accident in Reading, near Philadelphia, where a boiler had exploded on a newly made train. Lardner pronounced that the accident had been caused by lightning, which meant that Norris brothers were not personally liable for the accident. A committee of the Franklin Institute pointed out that there was no lightning present at that time and that the pumps had been faulty, the water indicator was ill-designed and the bridge-bands made of cast iron rather than wrought iron. The Coroner's inquest jury were persuaded by Lardner that the accident was an 'act of God' but the company were careful to design their later locomotives with wrought-iron bands.

References include:

• Kieran Byrne, 'Mechanics's Institutes in Ireland before 1855' (thesis, University College Cork, )
• Richard Fawkes 'Dion Boucicault'
• Norman McMillan 'Prometheus's Fire'
• Mark Blaug (ed.) 'William Whewell (1794-1866), Dionysius Lardner (1793-1859), Charles Babbage (1792-1871)'; J.N. Hays article 'Dionysius Lardner' in the Dictionary of National Biography
• Nora Crook's introduction to 'Mary Shelley's Literary Lives and other writings'
• R. John Brockmann "Twisted Rails, sunken ships, the rhetoric of nineteenth century Steamboat and Railroad Accident Investigation Reports, 1833-1879" (2005).
• Archives include Letters to Babbage, Letters in the Wellcome Library, including a 9 page autobiography in French, etc.
• Peckham, Morse. "Dr. Lardner’s Cabinet Cyclopaedia". The Papers of the Bibliographical Society of America 45 (1951): 37-58.