On the recommendation of
Charles Macklin, Johnstone and his wife were engaged by
Thomas Harris at
Covent Garden Theatre for three years, and Johnstone was well received on his début as Lionel on 2 October 1783.
William Shield operas His association with the operas of
William Shield commenced early. He appeared (as Dermott) with
John Edwin and
Charles Bannister in
The Poor Soldier (
John O'Keeffe's words) in 1783: in 1784 in Shield's
Robin Hood, his duet with Charles Bannister was 'unanimously encored', and later that year he sang "Let fame sound the trumpet" in Shield's
Fontainblau with great success: he and Bannister sang airs and duets at a dinner with Shield a few days afterwards. In
The Choleric Fathers (1785), (a Shield opera to a
Thomas Holcroft libretto) he was Don Fernando in a cast led by
John Quick. Shield's 1786 collaboration with O'Keeffe,
Love in a Camp saw him as Captain Patrick, and
Love and War (1787) (a
Robert Jephson script), again with Quick, gave him Captain Farquar. In
The Farmer (1787) Johnstone was Valentine to Charles Bannister's Farmer Blackberry, and in
The Highland Reel (1788) he was Sandy to Bannister's Serjeant Jack: in 1790 he was Colonel Lefort to Bannister's Peter I in
The Czar. All three were O'Keeffe texts set by Shield. 1790 also saw Johnstone as Raymond to Bannister's Duran in Shield's
The Crusade to a
Frederick Reynolds script. In the 1791 premiere of
The Woodman (text by
Dudley), with Quick, Bannister,
Incledon and Mrs Martyr, he played Captain O'Donnel 'with great vivacity and spirit, and gave his Irish ballad with peculiar felicity', though when recalled it was to watch mutely while Incledon gave encores. He was Sergeant of Grenadiers in
To Arms! in 1793, Dorimund in
Travellers in Switzerland in 1794, and in 1795, in the afterpiece
The Irish Mimic, or Blunders at Brighton (Shield and O'Keeffe) as Parrot he played an Irish character who failed completely to mimic certain birds and animals (but sang beautifully). Johnstone and William Shield, Charles Incledon,
Charles Dignum, Charles Bannister,
Charles Ashley and William Parke (oboeist) in 1793 formed themselves into 'The Glee Club', a set which met on Sunday evenings during the season at the Garrick's Head Coffee House in Bow Street, once a fortnight, for singing among themselves and dining together. 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. 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.
Samuel Arnold and others 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. He featured in
The Surrender of Calais (text by
George Colman the younger) at the Little Theatre,
Haymarket, in 1791, and was Harry Furnace in Warner's
The Armourer at Covent Garden in 1793. He appeared (as Captain O'Leary) with Incledon in
William Reeve's
British Fortitude and Hibernian Friendship in 1794, and was O'Curragh in Arnold's
Zorinski (1795) and Captain Macgallaher in his
Bannian Day (1796), again at the Haymarket. ''Britain's Brave Tars!'', a one-act farce by
Thomas Attwood to an O'Keeffe text, had Incledon as Lieutenant Tafferel and Johnstone as Pat Plunket (who sang "Brave Betty was a maiden") in the 1797 Covent Garden premiere. In 1798 he was The Bard in the premiere of Arnold's
Cambro-Britons at the Haymarket. Until 1803 he remained at Covent Garden, with occasional summer seasons at the Haymarket Theatre. 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. He took various other operatic tenor leads: however his singing voice did not wear well, and he gradually abandoned operatic parts. lacked the necessary whimsy for such roles. '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.' He excelled in roles such as Sir Lucius O'Trigger in
The Rivals, Major O'Flaherty (
The West Indian), Dennis Brulgruddery (
John Bull by
George Colman the younger), Looney Mactwolter (in
The Review, in which he was 'irresistibly ludicrous'), and Teague (
The Committee, by
Sir Robert Howard). He became known as "Irish" Johnstone, and, on his visiting Dublin in 1803, he was welcomed as a representative of authentic Irishmen on the stage. He was an Irish interpreter in Cherry's
The Travellers in January 1806. 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. ==Last years==