The son of a Dublin ironmonger, was born in Dublin. He started life as a small tradesman; but turning to politics, he became a member of the corporation of Dublin, and was popular for his denunciation of municipal corruption and his proposal of a boycott of English goods in Ireland, in retaliation for the restrictions imposed by the government on Irish commerce. In April 1780, Tandy was expelled from the Dublin volunteers (see Henry Flood) for proposing the expulsion of the Duke of Leinster, whose moderation had offended the extremists. He was one of the most conspicuous of the small revolutionary party, chiefly of the shopkeeper class, who formed a permanent committee in June 1784 to agitate for reform, and called a convention of delegates from all parts of Ireland, which met in October 1784.
Tandy persuaded the corporation of Dublin to condemn by resolution Pitt's amended commercial resolutions in 1785. He became a member of the Whig club founded by Henry Grattan; and he actively co-operated with Theobald Wolfe Tone in founding the Society of the United Irishmen in 1791, of which he became the first secretary. The violence of his opinions, strongly influenced by French revolutionary ideas, now brought Tandy to the notice of the government. In February 1792, an allusion in debate by Toler, the attorney-general, to Tandy's personal ugliness, provoked him into sending a challenge; this was treated by the House of Commons as a breach of privilege, and a Speaker's warrant was issued for his arrest, which he managed to elude till its validity expired on the prorogation of parliament. Tandy then took proceedings against the lord lieutenant for issuing a proclamation for his arrest; and although the action failed, it increased Tandy's popularity, and his expenses were paid by the Society of the United Irishmen.
Sympathy with the French Revolution was rapidly spreading in Ireland. A meeting of some 6000 people in Belfast voted a congratulatory address to the French nation in July 1791. In the following year, Napper Tandy took a leading part in organizing a new military association in Ireland modelled after the French National Guards; they professed republican principles, and on their uniform the cap of liberty instead of the crown surmounted the Irish harp. Tandy also, with the purpose of bringing about a fusion between the Defenders and the United Irishmen, took the oath of the Defenders, a Roman Catholic society whose agrarian and political violence had been increasing for several years; but being threatened with prosecution for this step, and also for libel, he fled to America, where he remained till 1798. In February 1798 he went to Paris, where at this time a number of Irish refugees, the most prominent of whom was Wolfe Tone, were assembled, planning rebellion in Ireland to be supported by a French invasion, and quarrelling among themselves. None of these was more quarrelsome than Napper Tandy, who was conceited and habitually drunk.
Wolfe Tone, who a few months before had patronizingly described Tandy to Talleyrand as "a respectable old man whose patriotism has been known for thirty years," was disgusted by the way Tandy persuaded the French authorities that he was a person of great wealth and influence in Ireland, at whose appearance 30,000 men would rise in arms. Tandy was not, however, lacking in courage. He accepted the charge of a corvette, the "Anacreon," placed at his disposal by the French government, in which, accompanied by a few United Irishmen, and supplied with a small force of men and a considerable quantity of arms and ammunition for distribution in Ireland, he sailed from Dunkirk and arrived at the isle of Aran, off the coast of Donegal, on September 16, 1708.
The local population showed no disposition to welcome the invaders. Napper Tandy, who was drunk during most of the expedition, took possession of the village of Rutland, where he hoisted an Irish flag and issued a proclamation; but learning the complete failure of Humbert's expedition, and that Connaught, instead of being in open rebellion, was perfectly quiet, the futility of the enterprise was apparent to the French if not to Tandy himself; having been carried on board the "Anacreon" in a state of intoxication, he sailed his vessel round the north of Scotland to avoid the English fleet, and reached Bergen in safety. Tandy then made his way to Hamburg with three or four companions. In compliance with a peremptory demand from the English government, and in spite of a counter-threat from the French Directory, the refugees were surrendered.
Tandy remained in prison till April 1801, when he was tried, pleaded guilty, and was sentenced to death; he was reprieved and allowed to go to France. This leniency may have been partly due to doubts as to the legality of the demand for his surrender by the Hamburg authorities; but the government was probably more influenced by Cornwallis's opinion that Tandy was "a fellow of so very contemptible a character that no person in this country (Ireland) seems to care the smallest degree about him." Moreover, Napoleon vigorously intervened on his behalf, and is even said to have made Tandy's release a condition of signing the Treaty of Amiens. Notwithstanding his vices and his lack of all solid capacity, there is no reason to suppose that Napper Tandy was dishonest or insincere; and the manner in which his name was introduced in the well-known ballad, "The Wearing of the Green", proves that he succeeded in impressing the popular imagination of the rebel party in Ireland. In France, where his release was regarded as a French diplomatic victory, he was received, in March 1802, as a person of distinction; and when he died his funeral was attended by the military and an immense number of the civil population.