Brian Mulligan debuted at the Metropolitan Opera while still a student at Juilliard, in December 2003 in Die Frau ohne Schatten. Since then, he has gone on to perform at some of the most important opera houses in the world, including San Francisco Opera (debut as Marcello in La Bohème), Opernhaus Zürich (debut as Yeletsky in Pique Dame), Lyric Opera of Chicago (debut as Enrico in Lucia di Lammermoor), Oper Frankfurt (debut as Prospero in Ades' The Tempest), Los Angeles Opera (debut as Prometheus in Die Vögel), New York City Opera (debut as Jake Wallace in La Fanciulla del West) and Houston Grand Opera (debut as Marcello in La Bohème).

In the 2015 - 16 season, Mulligan appeared as Sweeney Todd with San Francisco Opera, Paolo Albiani in Simon Boccanegra at the Metropolitan Opera, John Proctor in The Crucible at the Glimmerglass Festival, Tadeusz in The Passenger at the Theater an der Wien and Roderick Usher in Debussy's La chute de la Maison Usher at San Francisco Opera. In May 2016, Mulligan created the role of Jack Torrance in the world premiere of The Shining by Paul Moravec at the Minnesota Opera to rave reviews.

Brian Mulligan's debut album will be released in 2017 on Naxos. The album features two song cycles by Dominick Argento, The Andrée Expedition and From the Diary of Virginia Woolf. Timothy Long is the Pianist and Adam Abeshouse was the Producer.

Brian Mulligan has also performed with many of the finest orchestras in America, including the Cleveland Orchestra (A Sea Symphony), the Chicago Symphony Orchestra (Songs for Adam), the Houston Symphony (Paulus) and the Los Angeles Philharmonic (Mahler Symphony #8). A performance of Mahler’s 8th Symphony, conducted by Gustavo Dudamel, was recorded by Deutsche Grammophon and released on DVD in 2012.