Sierra studied composition in Europe, notably with György Ligeti in Hamburg (1979–1982), Germany. After his two-act opera El mensajero de plata, to a libretto by Myrna Casas, had premiered at the Interamerican Festival in San Juan on 9 October 1986, Sierra came to prominence in 1987 when his first major orchestral composition, Júbilo, was performed at Carnegie Hall by the Milwaukee Symphony Orchestra. (Júbilo had been premiered in Puerto Rico in 1985 by the Puerto Rico Symphony Orchestra conducted by Zdeněk Mácal; it was also performed in 1986 by the same forces conducted by Akira Endo.) For more than three decades his works have been part of the repertoire of many of the leading orchestras, ensembles and festivals in the USA and Europe. His Fandangos was performed at the opening night of the 2002 Proms, performed by the BBC Symphony Orchestra and televised throughout Europe.
Sierra is a professor at Cornell University in Ithaca, New York, where he teaches composition. See: List of music students by teacher: R to S#Roberto Sierra.
On February 2, 2006 Sierra's Missa Latina, premiered at the Kennedy Center, in Washington, D.C., conducted by Leonard Slatkin to considerable acclaim. The Washington Times judged it "the most significant symphonic premiere in the District since the late Benjamin Britten's War Requiem was first performed in the Washington National Cathedral in the late 1960s." On March 3, 2007, the Missa Latina was performed at the 51st Casals Festival in Sierra's homeland, Puerto Rico, where it was equally well-received.
Sierra's Concierto Barroco takes its inspiration from a scene in Alejo Carpentier's novel of the same name in which Handel and Vivaldi jam with a Cuban slave during the Venice Carnival. Sierra was commissioned by guitarist Manuel Barrueco to write a concerto that tried to capture what that might have been like. In Soundboard magazine, Eladio Scharron wrote that, "Sierra achieved – masterfully – a synthesis of a tradition of five centuries old... This work is truly a masterwork..."
Other commissioned works include: