In the 1960s, Brohn began his career as conductor for the Joffrey Ballet, American Ballet Theatre, and the Royal Ballet's tours of America. He then focused on orchestration during the next two decades, producing ballet scores for Agnes de Mille, Lar Lubovitch, Twyla Tharp, Susan Stroman and the American Ballet Theatre.[3]
In 1989, Brohn began to collaborate with Cameron Mackintosh, orchestrating approximately ten of his shows[4] also working on scores for musicals with Trevor Nunn and the Royal National Theatre, London.[3]
Additionally, Brohn has provided arrangements for Liza Minnelli (the complete orchestrations to her Minnelli on Minnelli tour, 1999–2000) Marilyn Horne, Renée Fleming, Frederica von Stade, Plácido Domingo Judy Collins and Jerry Hadley which have all been recorded. Brohn has also collaborated with conductors such as André Previn, John Williams and Keith Lockhart.
In 1987 Brohn was commissioned to make an adaptation of Prokofiev's film scores Alexander Nevsky and Ivan the Terrible which were later recorded onto CD. A Sony CD of his one-movement Suite from Bernstein's West Side Story for violin and orchestra, featuring Joshua Bell as violin soloist, was released in 2001. He also made arrangements for Sir James Galway on his Sony CD Wind Beneath My Wings. He has provided music for the Boston Pops Orchestra, including various adaptations of Christmas and theatre music, as well as for the Cleveland Orchestra and the Hollywood Bowl.
In 1996 Brohn was awarded an honorary Doctorate in Fine Art from Michigan State University and later presented a master class there entitled "The Future of Musical Theatre" in 2004.[2]
On 4 October 2009, the concert From Broadway to West End: By Special Arrangement was held in Bill's honour at the Theatre Royal Drury Lane in aid of the charity CLIC Sargent. The concert was conducted by his protégé Chris Jahnke and featured songs from Oklahoma!, My Fair Lady, Curtains, Wicked, Sweet Smell of Success, Miss Saigon, Mary Poppins, Ragtime, Oliver!, South Pacific, The Three Musketeers, Crazy for You and Carousel.