The protégé of the great Norman Treigle, Devlin began his career with the New Orleans Opera Association, where he debuted in a small role in La bohème (with Audrey Schuh as Mimì), in 1963.
He was subsequently seen in Werther (opposite Giuseppe di Stefano), Carmen, Ariadne auf Naxos, Aïda (as Amonasro, with Marisa Galvany) and, in 1999, returned for The Ballad of Baby Doe (as William Jennings Bryan).
At the New York City Opera, Devlin first sang the Ermitaño Ciego in the North American premiere of Ginastera's Don Rodrigo, conducted by Julius Rudel and staged by Tito Capobianco, in 1966.
Further appearances there included L'amour des trois oranges (as Léandre), Giulio Cesare (as Nireno, later the title role), The Crucible (opposite Chester Ludgin), Bomarzo (1968), Nine Rivers from Jordan (world premiere, by Weisgall), La bohème (as Colline), Le coq d'or, Lucia di Lammermoor, Manon, Œdipus rex, Le nozze di Figaro (in the title role), Mefistofele (with Harry Theyard), Louise, Carmen, Maria Stuarda, Susannah, Così fan tutte, Les contes d'Hoffmann, Faust (in Frank Corsaro's production) and Pelléas et Mélisande (as Golaud).
His other performances at that venue have included Hänsel und Gretel (as Peter), Les contes d'Hoffmann (as the Villains, opposite Plácido Domingo), Die Fledermaus (as Dr Falke), Bluebeard's Castle (with Jessye Norman), Salome (as Jokanaan), Samson et Dalila (as Abimélech), Faust, and Wozzeck (as the Doctor), altogether singing 229 performances in over thirty years with the Met.