The natural harbor of Colombo has been used by traders for thousands of years
Colombo's famed harbor was a natural stopping point for trade between the Middle East and Africa and Southeast and Far East Asia. Evidence points to Arab settlers from the 8th century using the port, but it is likely that it was a popular maritime destination much earlier. Seilan as Ceylon was sometimes called was probably used by ships sailing for China well before the 8th century.
The modern history of Colombo begins with the Portugese in the 16th century. The Dutch made it a major stopping point on the route to Indonesia. The British, who took over much of India in the 18th and 19th century, took charge of much of the island by the end of the 18th century. In World War II (see movie link below) it was a crucial port for the Allied efforts against Japan.