Johann Sebastian Bach is probably the most famous composer prior to Mozart, and with Handel, by far the most important German baroque composer. 

His first post  was church organist at Arnstadt, then in 1708, he became court organist at Weimar.  In 1718  he took the job of  musical director for Prince Leopold of Anhalt-Cothen.    In this post he composed most of his instrumental and orchestral music, since the prince belonged to the Calvinist church, whose austere services employed little music beside psalm singing.  In 1722 Bach left the services of Prince Leopold to become musical director for the city of Leipzig, and a teacher at St Thomas's School in Leipzig, and composer for the Church of St Thomas. Whist in Leipzig, Bach had  thirteen children by his second wife Anna Magdalena, although sadly only six grew to adulthood.

His output as a composer was enormous, displaying great vigour and invention within the northern European polyphonic tradition.  His music is an example of supreme craftsmanship - he mastered with mathematical precision the formal problems of counterpoint, and produced keyboard music in which as many as five separate lines of argument are simultaneously sustained.  He wrote 200 Cantatas and Oratorios, the six Brandenburg Concertos, and the 48 preludes and fugues in the "Well-tempered Clavier", the Mass in B minor, and the St.Mathew and St.John Passions.

Bach's work was not performed a great deal after his death, until midway through the 19th century, when in 1829 Mendelssohn conducted a performance of the St Mathew Passion in Berlin, starting a great revival of the master's works.