The earliest written mention of the port may be that of Fa-hsien, a Chinese traveler of the 5th century AD, who referred to the port as Kao-lan-pu. The Sinhalese called the port Kolamba, which the Portuguese thought was derived from the Sinhalese word for mango trees (kola, "leaves"; amba, "mango"). A more likely explanation is that kolamba was an old Sinhalese word meaning "port," or "ferry."

In the 8th century AD Arab traders settled near the site of the modern port. From the 16th century onward, the port was developed by the Portuguese, the Dutch, and the British, who each in turn established themselves on the island. In 1815, when the Sinhalese chiefs deposed the king of Kandy in the heart of Ceylon (now Sri Lanka) and ceded his territory to the British, Colombo became the capital of the island. Western influence in the city has diminished since Sri Lanka gained its independence in 1948.

The oldest districts of the city, which are nearest to the harbour and north of Beira Lake, are known as the Fort and the Pettah (a name deriving from the Tamil word pettai, meaning "the town outside the fort"). The Fort is still a focal point of government and commercial activity, although less so than in the past. The Pettah has become a district of small shops, markets, and sidewalk stalls, with all the characteristic features of an Oriental bazaar. The built-up area of Cinnamon Gardens, which is south of Beira Lake, was an important cinnamon-growing region during the Dutch period.