American baritone Robert Mellon has been praised not only for his "glorious voice that burst[s] out brimming with life and vigor," but also for having "immense skill as an actor."

2024 engagements included Detroit Opera as Terry in Breaking the Waves, Florida Grand Opera for Tonio in Pagliacci and Opera Theatre of St. Louis for roles in La bohème and Galileo, followed by Bartolo in Il barbiere di Siviglia with Indianapolis Opera. In 2025, Mr. Mellon debuts the role of Sharpless in Madama Butterfly with Anchorage Opera, returns to Opera Tampa for Doctor Malatesta in Don Pasquale and entertains St. Louis audiences as Frank in Die Fledermaus and Quince in Midsummer Night’s Dream.

In the 2022–2023 season he returned to Livermore Valley Opera to sing George in Of Mice and Men and made several house debuts: Winter Opera St. Louis as Leporello in Don Giovanni, Lyric Opera of Kansas City as Escamillo in Carmen and Tulsa Opera as Taddeo in L'italiana in Algeri. Robert returned to Pensacola Opera as Marcello in La bohème and Syracuse Opera/Tri-Cities Opera for the title role in Le nozze di Figaro. Additional engagements included a role debut as Dandini in La Cenerentola with Opera Maine, where he previously performed Figaro in Le nozze di Figaro and Papageno in Die Zauberflöte.

Robert returned to Opera de Oviedo as Marcello in La bohème and Gubetta in Lucrezia Borgia, having previously performed with the company as Simone in Eine Florentinische Tragödie and Tonio in Pagliacci. He made his debut with Opera Philadelphia as Marullo in Rigoletto and Pensacola Opera as Papageno in Die Zauberflöte. With In-Series Opera, Mr. Mellon sang the role of Iago in Otello. A frequent performer with Union Avenue Opera, he won spectacular reviews in the title role of Falstaff, where he sang the "honor aria" with an "authority and comic force" that made it a "genuine showstopper." That performance won him the St. Louis Theater Circle Award for Outstanding Achievement in Opera.

Other outstanding credits include San Diego Opera as Schaunard in La bohème, Las Vegas Opera as Malatesta in Don Pasquale, Union Avenue Opera as Figaro in Il barbiere di Siviglia, and Syracuse Opera in the roles of Don Alfonso in Così fan tutte and Leporello in Don Giovanni. He sang in the world premiere of Dear Erich with New York City Opera and performed as Willem/Usher in the American premiere of Glass's The Trial with Opera Theatre of Saint Louis. In a double bill with Livermore Valley Opera, he sang the title role of Gianni Schicchi and delivered a performance of Simone in Eine Florentinische Tragödie that won the company the American Opera Award. According to the San Francisco Chronicle, he sang Simone in "robust, dark-hued tones that consistently rang out at the forefront of the musical texture...with a steely vocal edge that never interfered with the beauty and flexibility of his singing."

Concert work includes the Mozart and Duruflé Requiems, Dvorák's Te Deum, Stravinsky's Pulcinella, Rossini's Petite Messe Solonelle, Bach's Magnificat, and Mussorgsky's Songs and Dances of Death. In a concert performance of La Belle et le Bête with Present Music he sang the roles of Le Père, Ludovic, and L'Usurier.