Neilson is born in Ballyroney, County Down, the son of Presbyterian minister Alexander, and Agnes Neilson. He is educated locally. He is the second son in a family of eight sons and five daughters. At the age of 16, he is apprenticed to his elder brother, John, in the business of woollen drapery in Belfast. Eight years later he establishes his own business.
Despite his commercial success, Neilson is naturally drawn to politics and is early on a member of the reformist Irish Volunteers movement. In 1791, inspired by the French Revolution, he suggests to Henry Joy McCracken the idea of a political society of Irishmen of every religious persuasion. He establishes the Society of United Irishmen in Belfast. The following year he launches the newspaper of the United Irishmen, the Northern Star, which effectively throws away his fortune. As its editor he is a high-profile target for the authorities and is prosecuted for libel several times, being twice imprisoned between 1796 and 1798.
Along with several other state prisoners, Neilson is released in February 1798 following several petitions by influential friends, on grounds of bad health. Upon release he immediately involves himself in the United Irishmen aligning with the radicals among the leadership who are pressing for immediate rebellion and oppose the moderates who wish to wait for French assistance before acting.
The United Irishmen are however, severely penetrated by informants who keep Dublin Castle abreast of their plans and discussions. In March 1798, information of a meeting of the United Irish executive at the house of Oliver Bond leads to the arrest of most of the leadership, leaving Neilson and Lord Edward FitzGerald as the only figures of national importance still at liberty. They decide to press ahead as soon as possible and settled on May 23 as the date for the rebellion to begin.
As the date looms closer, the authorities go into overdrive to sweep up the rump leadership and on May 18 Lord Edward is betrayed in his hiding place and critically wounded while resisting capture. Neilson, now with responsibility for finalising plans for the looming rebellion, decides that Fitzgerald is too valuable to do without, and decides to try and rescue him from Newgate Prison in Dublin. Wary of confiding his plans too early for fear of betrayal, Neilson goes on a reconnaissance of the prison but is spotted by chance by one of his former jailers and after a fierce struggle, he is overpowered and dragged into the prison.
Neilson is indicted for high treason and held in Kilmainham Gaol with other state prisoners for the duration of the doomed rebellion outside. After the execution of Oliver Bond, and the brothers John and Henry Sheares, Neilson and the remaining prisoners agree to provide the authorities with details of the organisation of the United Irishmen, plans for the rebellion, etc. in return for exile.
Following the suppression of the rebellion, Neilson is transferred to Fort George in Inverness-shire, Scotland, and in 1802 he is deported to the Netherlands. From there he makes his way to the United States, arriving in December 1802, and settling in Poughkeepsie, New York. He has little time to enjoy his liberty before his sudden death on August 29, 1803 of yellow fever, or possibly a stroke.
Nielson is not idle during his short life in America as he completes plans to start a new evening newspaper in Poughkeepsie and also has plans in the works to establish a version of the Society of United Irishmen in the United States. Since his death, his remains have been moved to three different cemeteries before coming to rest in the Poughkeepsie Rural Cemetery in 1880.
(Photograph: (c) National Museums Northern Ireland; Supplied by The Public Catalogue Foundation)