Jackson was born in Newtownards, County Down, in 1737. Much is unclear about his early life. He studies at Oxford and became an Anglican curate. In the 1760s, he moves to London, where he preaches at the Tavistock Chapel and St. Mary-le-Strand. Although he gains some popularity as a preacher, he remains unbeneficed and eventually turns to journalism to support himself.
In 1766, Jackson becomes the editor of The Public Ledger. Under his editorship, the London paper becomes increasingly strident and oppositional in its politics. He is forced to flee to France in April 1777 to avoid a trial for libel that the popular actor and playwright Samuel Foote had initiated. He does not have to stay long in exile because Foote dies on October 21 of that same year.
After Foote’s death, Jackson returns to England. He resumes his political activities by publishing The Constitutions of the Several Independent States of America in 1783, with a dedication to the opposition leader, William Bentinck, 4th Duke of Portland. But the following year, he is secretly hired by the Prime Minister, William Pitt the Younger, to support the government in The Morning Post. Publishing anonymously, he attacks his former allies with his usual vehemence until he is discovered and is soundly damned for his apostasy and finds himself generally excluded from English politics.
Jackson’s next appearance in the public results in yet another scandal. In 1787 he joins forces with “Gentleman” John Palmer. Their goal is to build a new theatre in London. Jackson and Palmer persuade investors to sink more than eighteen thousand pounds into the construction of the Royalty Theatre. However, while there is no law against building a theatre in London, there is a law against operating one without the Lord Chamberlain‘s authorisation. Jackson and Palmer have no such authorisation so the theatre is shut down after just one night. The duped investors initiated legal action. Jackson again flees to France, where he arrives on the eve of revolution.
During his stay in Paris, Jackson is swept up in the revolutionary fervour and becomes involved with the radical British expatriate set there. Swept up in the general arrest of British subjects in 1793, he is released from prison on the strength of his radical commitments. Upon his release, he becomes inspector of horses for Meaux and later in 1793 is commissioned as a spy for the French. Nicholas Madgett, an Irishman who works in the Marine Ministry, recruits Jackson to go to England and Ireland to assess the public’s inclination towards armed revolution.
Jackson arrives in London in early 1794 and becomes reacquainted with John Cockayne, a lawyer he had met two decades earlier. He reveals his mission to Cockayne, who promptly reveals it to the Prime Minister out of fear of being tried for treason himself. When Jackson leaves London for Dublin, he is accompanied by Cockayne. In Ireland they meet with several radical leaders of the Society of United Irishmen, including Theobald Wolfe Tone, James Reynolds and Archibald Hamilton Rowan. Hamilton Rowan, in particular, is tempted by Jackson’s talk of French assistance, and persuades Tone to write up a report for the French, indicating Irish willingness to rise up. Jackson makes the fatal mistake of placing Tone’s report and other letters in the public mail, where they are seized by the authorities. This seizure leads to Jackson’s arrest on April 28, 1794.
Jackson remains in prison for a year before his trial takes place. The delays are at his request, allowing him time to assemble a defence and procure witnesses. During his imprisonment, he writes his last work, Observations in Answer to Mr. Paine’s Age of Reason (1795). His trial takes place in Dublin on April 23, 1795, and he is found guilty. One week later, on the morning of his sentencing hearing Jackson steps into the dock looking terribly ill. As his lawyers make drawn out speeches, hoping to avoid judgment on the technicality of an improperly filed indictment, Jackson’s condition steadily worsens. The judges order that a chair be provided for him and ask that a doctor attend him. He then collapses and dies. An autopsy finds that Jackson had ingested a large quantity of a “metallic poison.” This is likely administered by his second wife, but the inquest pointedly refuses to assign blame.
The effect of Jackson’s suicide is that he had not actually been pronounced guilty of treason by the court, and so his family can inherit his goods and a pension.