Castel was born in Lisbon, Portugal, raised in Venezuela by multilingual parents and a German nanny, and attended a French school. He moved to New York City at the age of 16 to pursue a singing career and in 1958 became the first winner of the "Joy in Singing" award, which launched his career with a New York Town Hall debut recital. In 1958 he made his debut with the Santa Fe Opera in Verdi's Falstaff then portrayed Joseph in the world premiere of Carlisle Floyd's Wuthering Heights. Soon afterwards, he made his debut with the New York City Opera and then the Metropolitan Opera in 1969, where he has sung for over 40 years with over 200 operatic roles in his complete repertoire.
Castel has served as a staff diction coach at the Metropolitan Opera for over 30 years. A polyglot, he speaks Portuguese, German, French, Spanish, Italian and English with native or near-native fluency. He is on the faculties of The Juilliard School of Music and Mannes College The New School for Music in New York, and is a noted lecturer and teacher at universities and conservatories throughout the world. His language and diction classes are taught at The Juilliard School, Manhattan Music Division, Eastman Music Division, Indiana University, New York University, Metropolitan Opera, New York City Opera, Chicago Opera Center, Pittsburgh Opera Center, The Opera NUOVA Vocal Intensive Program in Edmonton, Alberta, and A.I.M.S. (Graz, Austria).
Since 1992 he has been Adjunct Professor of Music at Boston University. Castel and his wife, director and voice teacher Carol Cates Castel, also teach on the faculty of the Spoleto Vocal Arts Symposium in Spoleto, Italy, recognized as one of the finest training programs in Italy for singers. The Castels are founders of the New York Opera Studio (NYOS), a training program for young artists that hosts summer intensives at Vassar College.