Parsons is born at Birr Castle, Parsonstown, King’s County (now County Offaly), the son and heir of the astronomer William Parsons, 3rd Earl of Rosse, who built the “Leviathan of Parsonstown” reflecting telescope, largest of its day, and his wife, the Countess of Rosse (née Mary Field), an amateur astronomer and pioneering photographer. He succeeds his father in 1867 and is educated first at home by tutors, like John Purser, and after at Trinity College, Dublin and the University of Oxford. He is the brother of Charles Algernon Parsons, inventor of the steam turbine.
Parsons serves as the eighteenth Chancellor of Trinity College, Dublin between 1885 and 1908. His father serves as the sixteenth Chancellor. He is Lord Lieutenant of King’s County and Custos Rotulorum of King’s County from 1892 until his death. He is also a Justice of the Peace for the county and is appointed High Sheriff of King’s County for 1867–68. He is knighted KP in 1890.
Parsons also performs some preliminary work in association with the practices of the electrodeposition of copper sulfate upon silver films circa 1865 while in search of the design for a truly flat mirror to use in a telescope. However, he finds it impossible to properly electroplate copper upon these silver films, as the copper contracts and detaches from the underlying glass substrate. His note has been cited as one of the earliest confirmations in literature that thin films on glass substrates experience residual stresses. He revives discussion in his work Nature’s August 1908 edition after witnessing similar techniques used to present newly-devised searchlights before the Royal Society.
Although overshadowed by his father (when astronomers speak of “Lord Rosse”, it is almost always the father that they refer to), Parsons nonetheless pursues some astronomical observations of his own, particularly of the Moon. Most notably, he discovers NGC 2, a spiral galaxy in the constellation Pegasus.
Parsons is elected a Fellow of the Royal Society in December 1867 and delivers the Bakerian lecture there in 1873. He is vice-president of the society in 1881 and 1887. From 1896 he is President of the Royal Irish Academy. In May 1902 he is at Caernarfon to receive the honorary degree LL.D. (Legum Doctor) from the University of Wales during the ceremony to install the Prince of Wales (later King George V) as Chancellor of that university.