Despite concerns that his handsome but compact baritone could not withstand the rigors of Wagner's The Flying Dutchman, Wotan or Hans Sachs, Thomas Stewart enjoyed a long career in these leading roles, making up in adherence to a firm legato and crisp enunciation what he lacked in sheer vocal weight. Although he intelligently managed the lowest notes of these long-ranged parts, he was a true baritone, not a bass baritone; the extra freedom he possessed in his top range remained with him and helped him through the most fatiguing passages. Not physically large, Stewart carried himself with such authority that he conveyed Wotan's wrath and Vanderdecken's torment with more than ample power. He appeared frequently with his wife, soprano Evelyn Lear, in recital and concert. Apart, they were each artists of great distinction; together, they made appealing musical partners, distinguished and communicative.
After study at the Juilliard School of Music, Stewart made his 1954 stage debut in the American premiere of Richard Strauss' Capriccio, singing theater director La Roche, a bass role. In the spring of 1954, he joined the New York City Opera, singing the Commendatore in Don Giovanni and the First Soldier in Salome (more bass parts). Chicago's Lyric Opera offered Stewart further opportunities for bass roles when he joined the company in the fall of 1954. There he sang Baptista in Vittorio Giannini's The Taming of the Shrew, Raimondo in Lucia di Lammermoor, and Angelotti in Tosca. Europe seemed to offer more promise in 1956; after smaller roles, Stewart graduated to larger parts in the baritone and bass baritone registers. In 1958, he joined the Städtische Oper (now Deutsche Oper) in Berlin, making his debut as Escamillo. The Toreador was also the role for his Covent Garden debut in 1960. Stewart's first season at the Bayreuth Festival was 1960 when he appeared as Donner, Gunther, and Vanderdecken. His success that year led to a close relationship with the festival over the ensuing decade and a half; he eventually took on the role of Wotan. At the Paris Opéra, he made his debut in 1964 singing Gunther.
San Francisco played host to Stewart for several seasons, beginning in 1962 when he was kept busy with Rodrigo (Don Carlos), Valentin, Escamillo, and Ford. While his Toreador was found somewhat small-scaled, his Rodrigo and Ford aroused great enthusiasm. In the latter role, he played against Geraint Evans' feisty Falstaff and eloquently conveyed Ford's frustration and jealousy while singing handsomely. In 1981, Stewart sang the American premiere of Aribert Reimann's Lear at San Francisco. His Metropolitan Opera debut came on March 9, 1966, when he repeated his Ford, beginning 14 seasons with the company. His roles there ranged from Wotan, Hans Sachs, and Amfortas through Jochanaan and Goloud to Balstrode in Peter Grimes. All were sung with craft and expressiveness, even when they fell short of being definitive.
In Europe, Stewart was Herbert von Karajan's Wotan of choice for Walküre and Siegfried in his Ring cycle, both in recording and in live performance at Salzburg. During the 1960s and 1970s, many of the baritone's other leading roles were captured in live recordings, notably his Amfortas, Vanderdecken, Gunther, Telramund, and Orest. At Santa Fe in 1967, he portrayed Cardillac in the American premiere of Hindemith's opera.