In 1958 Sliker made his debut at Carnegie Hall in a sold out concert of Bach chamber music where he performed cantata Tritt auf die Glaubensbahn, BWV 152 under the baton of Václav Nelhýbel.
He notably portrayed one of the Seven Deadly Sins in the world premiere of Hugo Weisgall's Six Characters in Search of an Author on April 26, 1959[5] and sang in the 1959 production and cast recording of Douglas Moore's The Ballad of Baby Doe with Beverly Sills in the title role.
In 1960 Sliker was a soloist in a performance of Wolfgang Amadeus Mozart's Litaniae de venerabili altaris sacramento with the Boston Symphony Orchestra under conductor Hugh Ross at the Tanglewood Music Festival.
He made his debut with the company on November 29, 1961, as one of the noblemen in Richard Wagner's Lohengrin with Sándor Kónya in the title role, Ingrid Bjoner as Elsa, Margaret Harshaw as Ortrud, Randolph Symonette as Telramund, Jerome Hines as King Heinrich, and Joseph Rosenstock conducting.
His final and 509th Met appearance was as Guccio in Puccini's Gianni Schicchi on December 16, 1989, with Bruno Pola in the title role, Hei-Kyung Hong as Lauretta, and James Levine conducting.