Duncan's farm was named Alderlea, and this was the first name of the adjacent settlement. In August 1886, the Esquimalt and Nanaimo Railway was opened. No stop had been scheduled at Alderlea for the inaugural train bearing Sir John A. Macdonald and Robert Dunsmuir. However, at Duncan's Crossing, the level crossing nearest Alderlea, a crowd of 2,000 had assembled around a decorated arch and the train came to an unplanned halt, quite literally putting it on the map.
In the early 1900s, Duncan's Chinatown was the social centre for the Cowichan Valley's Chinese population. Chinatown was concentrated in a single block in the southwestern corner of Duncan. At its largest point, Duncan's Chinatown included six Chinese families and 30 merchants supplying loggers, millworkers and cannery and mine workers. As immigration laws became more restrictive, businesses closed and the buildings became run down. The city tore the buildings down in 1969 to build a new law courts complex. Some materials from the original buildings was used at Whippletree Junction.
In the 1980s, Duncan was linked to the 1985 bombings at Narita Airport in Japan and aboard Air India Flight 182, Canada's largest murder case. Resident Inderjit Singh Reyat purchased bomb parts and a radio used to conceal a bomb at Duncan stores. Less than two weeks prior to the bombings, Reyat and suspected Air India mastermind Talwinder Singh Parmar were observed testing explosives in the woods outside of Duncan by the Canadian Security Intelligence Service (CSIS).
Duncan has a large First Nations community and is the traditional home of the Cowichan Tribes, who are the largest band among the Coast Salish people. The Coast Salish women of the Cowichan Tribes are makers of the world famous Cowichan Sweaters.
Duncan boasts one of the largest Farmers' Markets on Vancouver Island. Held just North of town at the BC Forest Discovery Centre every Saturday from 9:00 - 2:00. The Market offers local produce, nursery products, and crafts.
Duncan is home to the BC Forest Discovery Centre. Before the U.S.-Canada softwood lumber dispute, Duncan and the whole Cowichan Valley were a thriving lumber centre in British Columbia.
Duncan has the world's largest ice hockey stick, officially recognised by Guinness World Records on July 14, 2008, which is on display on the side of the local arena (Cowichan Community Centre). The stick was made specifically for Expo 86 in Vancouver.
Duncan has one public secondary school, Cowichan Secondary School, as well as several elementary and middle schools. It also has one private secondary university preparatory school, Queen Margaret's School for girls, established in 1921 which has a co-ed junior school included. There is also an independent Catholic school, Queen of Angels which continues up to Grade 9. The head offices for School District 79 Cowichan Valley are also located in Duncan.
Duncan is the home city of the British Columbia Hockey League's Cowichan Valley Capitals, who play in the Cowichan Community Centre. The Cowichan Valley is known for the many fine athletes it produces, and due to its temperate weather, most sports are able to be played year-round.