Keiser's personality was both extravagant and self-indulgent, while his work habits were exacting. His later operas show increasingly the influence of Italian opera on his own works; he adapted aria forms, scenic structures, and much of his musical language from the Italians. He introduced, in the 1720s and 1730s, the practice of interpolating into his German operas, pre-composed Italian arias. Because Italian music was so popular and well loved in Hamburg, this soon became a standard practice with all German composers. The interpolations were many, but the other composers were always acknowledged in the libretti.
Born in Teuchern, near Weißenfels, Reinhard Keiser was the son of the organist and composer Gottfried Keiser. His father abandoned his mother and her two sons while Reinhard was still a youth. Reinhard studied music at the Thomasschule with Johann Schelle and may have also studied composition with Johann Kuhnau. His first formal position was as a court composer in Brunswick. Johann Kusser was in charge of the Brunswick opera, and it was due to his early influence that Keiser began having operas produced for the theaters in Brunswick and Hamburg. By 1697, Keiser had already written several operas for the Hamburg stage, and moved there permanently. In 1703 he began to manage the Hamburg Opera House, also known as the Gänsemarkt Theater of Hamburg, but financial difficulties soon followed, supposedly due to his own extravagances. Between the years 1705 and 1718, he produced countless new works. But when management changed hands, he was dropped as musical director, and did not again work steadily for any theater until 1722. He was brought back to the Gänsemarkt under Telemann, and the two composers worked side by side, with Keiser the dominant force in operatic composition and production. On 28 December 1728, Keiser became the Kantor of the Hamburg Cathedral, and retired from operatic composition altogether. He died on 12 September 1729.