In July 1839 Smythe became secretary of the Royal Artillery Institution at Woolwich, and filled the office until he embarked for St. Helena in December 1811 to take charge of the observatory at Longwood, and to carry out magnetical and meteorological observations under the direction of Captain (afterwards General Sir) Edward Sabine.
In January 1850 he was appointed by the Marquis of Anglesey to take charge of young officers of artillery on first joining at Woolwich, and to supervise their instruction.
Having a good knowledge of French and German, Smythe was selected in October 1854 to superintend the execution of contracts for arms in Belgium and Germany.
While still holding this appointment he was withdrawn temporarily from its duties by Lord Panmure, in January 1856, to act as a member of the royal commission sent to France, Russia, Austria, and Italy, to report on the state of military education in those countries, and to consider the best mode of reorganising the system of training British officers of the scientific corps.
In the end the report was drawn up by Lake and the secretary, Smythe signing "for the history and descriptions of foreign military schools only".
He married, on 15 December 1857, at Carnmoney, Sarah Maria, second daughter of the Reverend Robert Wintringham Bland, JP There was no issue of the marriage.
Smythe, accompanied by his wife, left England on 16 January 1860, taking with him complete sets of magnetical and meteorological instruments and charts.
Foreseeing a tolerably long detention in the islands, Smythe brought with him to Levuka materials for a small house, which was erected, and part of it was fitted as an observatory.
His wife wrote a pleasant account of the expedition in a series of letters to friends at home, which was published in 1864 as Ten Months in the Fiji Islands with coloured illustrations and maps.
To it Smythe contributed the introduction, an account of an excursion to Namosi in Viti Levu, and the appendix, containing his instructions and report, together with his magnetical and meteorological observations and remarks upon the Melanesian mission.
Smythe's latter years were chiefly given to an earnest advocacy of home rule for Ireland so far as it was compatible with union with Great Britain.