[1] Barry Millington from The Guardian summarised: "Though he made his career as a comprimario tenor, putatively subordinate to the principal roles, his incisively focused voice, hyperactive stage presence and musico-dramatic intelligence frequently conspired to make him the main attraction.

While head of department at the first sixth-form college in the country in Doncaster (1966 to 1969), Clark decided to sing just as a hobby, but wrote to Sadler's Wells Opera in London for a voice appraisal, which resulted in him meeting a senior member of their music staff, Tom Hammond, and a recommendation to take weekly lessons with Bruce Boyce.

[1] In Wexford Clark was noted by an agent who arranged an audition with Richard Bonynge; although apparently disinterested, a few days later[5] Bonynge accepted him for a charity gala held at the Royal Opera House in London on 25 January 1975 with Joan Sutherland, Heather Begg, Clifford Grant and others in aid of Darwin, which had been devastated by Cyclone Tracy.

[4] The Covent Garden concert was heard by Scottish Opera's general manager Peter Hemmings, who auditioned him and offered him a full-time contract with the company; this meant a 70 per cent drop in salary.

[1][7] Reflecting on his time at the company, Clark recalled "I was a total, raw beginner, a complete unknown who knew nothing about music, nothing about opera and I learnt the trade as I went along".

[2] He came to realize however that it was "abundantly clear that my voice was not right for [Italian repertoire] I didn't have the sweetness" and that "the roles I was most interested in were those that had some sort of substantial character that I could get my teeth into".

His performance as Loge was described: "Preening narcissistically ... in black leather, with a sculpted, blond, 1980s David Bowie-style hairpiece, he deployed a pungent, edgy tone very effectively to display his contempt for Wotan and the other gods.

[8][9] He participated there in the world premiere of Corigliano's The Ghosts of Versailles in 1991, described as "the slithery, acrobatic villain" Bégearss, alongside Teresa Stratas as Marie Antoinette.

[11] Clark performed at the Royal Opera House first as Mime in 1995, in the controversial production directed by Richard Jones, leaving a lasting impression o a reviewer from The Guardian who described the "terrifying sight of Graham Clark as the dwarf Mime, deranged by power lust, staring at the audience with penetrating, unblinking eyes and wearing the dress that belonged to Siegfried's mother, Sieglinde".

[12] He appeared there as the Marne Seargent in the world premiere of Iain Bell's In Parenthesis, a 2016 coproduction with the Welsh National Opera.

He performed at Oper Frankfurt as the Fool in Reimann's Lear and in Janácek's The Makropoulos Case, and returned in the 2010s as Monsieur Taupe in Capriccio by Richard Strauss.