It is said that on his discharge his colonel recommended him for his singing in a letter to Thomas Ryder, manager of the Smock Alley Theatre in Dublin.
[5] In The Choleric Fathers (1785), (a Shield opera to a Thomas Holcroft libretto) he was Don Fernando in a cast led by John Quick.
A project to erect a bust to Dr Thomas Arne, which this group proposed to fund by charitable performances, was vetoed by the management of Covent Garden.
[15] It is told that Johnstone was a regular drinking companion of Charles Incledon's at 'a public house of the lowest class', The Brown Bear in Bow Street.
[16] Johnstone was also associated with the operas of Samuel Arnold, and his appearance in 1789 performances of Inkle and Yarico (1787), supporting Mrs Billington, is described by Parke.
[17] He featured in The Surrender of Calais (text by George Colman the younger) at the Little Theatre, Haymarket, in 1791,[18] and was Harry Furnace in Warner's The Armourer at Covent Garden in 1793.
He performed Macheath in the Beggar's Opera, and once appeared as Lucy at the Haymarket in a production with John Bannister, when the male and female parts were reversed.
'His perfect brogue, his exquisitely comic manner, and his agreeable voice in singing, formed an irresistible charm, which was much enhanced by his handsome person and free military address.
[2] Johnstone joined Joseph George Holman's protest against the new regulations at Covent Garden Theatre, and accepted an engagement at Drury Lane in 1803, where he often shared the stage with John Bannister.
He acted at Drury Lane for the rest of his career, though he returned to Covent Garden as Sir Callaghan on the occasion of Charles Mathews's benefit, 8 June 1814, and again in 1820.
[2] Johnstone married, first, Ann Maria, the daughter of Colonel Poitier, governor of Kilmainham gaol; she was an operatic singer, and instructed him in music, but they separated a few months after marriage.