Willis was born on February 23, 1968, in Bethesda, Maryland. Her brother is U.S. conductor Alastair Willis. Sarah's upbringing was truly international, including stints in Moscow, London, and Tokyo. She took piano lessons as a girl, but when she was 14 and had a single lesson on the French horn, she immediately switched to that instrument. Living in Britain, she joined a youth orchestra and then enrolled at the Junior Department of London's Royal College of Music. Willis went on to the Guildhall School of Music and Drama, studying with Anthony Halstead and Jeff Bryant. After hearing the Berlin Philharmonic Orchestra live, she moved to Berlin, where she took lessons with Fergus McWilliam and won a place in the orchestra of Berlin's Staatsoper, conducted at the time by Daniel Barenboim. She made her recording debut in 1999 on CPO, performing the Concerto for two horns and orchestra of Antonio Rosetti with the SWR Baden-Baden and Freiburg Symphony Orchestra under conductor Holger Schroter-Seebeck.
In 2001, Willis auditioned for and won a place in the Berlin Philharmonic. She has made solo appearances with the Chicago Symphony, London Symphony, and Sydney Symphony Orchestras, among others, and is also an enthusiastic chamber music player. Willis has been unusually active in promoting classical music online, hosting her own streaming video channel, Horn Hangouts, serving as an interviewer for the Berlin Philharmonic's Digital Concert Hall, and acting in 2011 as a mentor and presenter for the YouTube Symphony Orchestra. Willis made several more recordings of Rosetti's music for CPO. She then moved to the Alpha label in 2020 for the album Mozart y Mambo, recorded with the Havana Lyceum Orchestra. In 2021, she was appointed a Member of the Order of the British Empire and released the album Horn Discoveries. Mozart y Mambo was reissued with new content by Alpha in 2022. ~ James Manheim, Rovi