Edward Thomas Gillon was born in Douglas, Isle of Man, England, probably on 21 January 1842, and baptised on 16 March. He was the son of Sarah Heron and her husband, Patrick Thomas Gillon, a merchant. He attended Forester's school in Douglas before the family emigrated to Dunedin, New Zealand, in 1851. Thereafter he was tutored by his mother. Gillon began working as a journalist for the Otago Witness in the Otago goldmining townships, and in 1861 became chief reporter for the newly established Otago Daily Times. In 1862 an attack of fever forced him to leave Dunedin, and he became clerk to the Bench in Tokomairiro, South Otago. In Milton in 1864 he helped to establish the Bruce Herald, which he edited until he founded the Bruce Standard. He married Isabella Jackson Miller at Tokomairiro on 8 February 1865.
Gillon became a parliamentary reporter in Wellington in 1867, working for several newspapers; he joined Hansard briefly, then worked for the Evening Post. In 1872 he became manager of a press association, and on its sale, editor of the Evening Post. He resigned this position in 1875 to contest, unsuccessfully, the parliamentary seat of Wellington Country, having been similarly unsuccessful in 1871. From 1875 to 1876 Gillon served on the Wellington City Council and as a provincial council member for Wellington City. He managed the United Press Association for five years from its formation in 1879.
In 1884 Gillon returned to the editor's chair in the Evening Post. His literary skills and his strong personality matured so that he became one of the most influential journalists in the country. He was recognised as one of New Zealand's best Shakespearian scholars, was a clever horseman, an enthusiastic volunteer in the services, and a good cricketer.
Gillon is remembered above all for his fight to uphold the principle that a journalist who acquires information honourably but confidentially should not disclose it or reveal his source. In 1885, in a case that scandalised Wellington, one prominent citizen was charged with aggravated assault on another man of equal social prominence. Gillon, who had been given information confidentially by the accused during his inquiries as a journalist, refused, first in the Resident Magistrate's Court and later in the Supreme Court, to divulge what he had been told. Subpoenaed by the prosecution, he said from the witness box that he 'declined to be forced into the position of a private detective, spy or informer'. When the presiding magistrate warned him that he was liable to imprisonment, he remained adamant (he was prepared to edit the Post from prison), and the prosecution did not force the issue.
In 1894, after the Evening Post had published details of a letter of resignation sent by Colonel F. J. Fox, commandant of the New Zealand forces, to the premier, Richard Seddon, Gillon refused to appear before a royal commission set up to investigate the leak. Gillon said that no penalties would induce him to disclose his source: 'I hold this to be a point of journalistic honour from which no departure is possible'. The commission, as the courts of law had done earlier, reluctantly respected his fearless stand. Gillon was a founding member, and the first president for three years, of the New Zealand Institute of Journalists.
Gillon often came to the office at 8.30 a.m. with his editorial, written before he got out of bed, ready for the printer. His writings were read eagerly throughout the colony. Gillon's editorial style was slashing and uncompromising, his meaning never in doubt. Yet, fighter as he was, he was scrupulously fair, and both as a journalist and as a man he was incorruptible. He had a kind heart and a tender sympathy for the suffering; but if, in his opinion, the occasion demanded it, he could write 'with vitriol almost as easily as with the milk of human kindness'.
Gillon died at Wellington on 19 April 1896 after a severe illness. He had asked that his funeral at Bolton Street cemetery be of the simplest, with no Masonic regalia despite his high standing in the order. He was survived by three sons and two daughters. Isabella Gillon had died on 11 December 1881.