Cassilis was named after a small township in Scotland. In the 1800s at the peak of the gold boom, it was once a thriving mining centre home to more than 500 people.

Gold bearing quartz reefs were first discovered in 1885 at Long Gully, and by 1889 there was a battery in operation as well as a general store, butchery, bakery and Mechanics' Institute hall. A private school had already been operating when a government school commenced in the hall in 1890. The schools produced exceptional results, with pupils under headmaster McDonald gaining the highest percentage of scholarships in the State for several years. Two hotels, a coffee palace, hairdressers, bookmakers, two bank agencies and two post offices were also in existence before.

The first hydroelectric power station in Victoria was established at Victoria Falls to provide power and light for the mine.
Of the reef mines, the Mt Hepburn and King Cassilis were the largest. The Mt Hepburn mine was an immense formation nearly a kilometre in length and as much as 15 metres wide. However the ore obtained was heavily mineralised, and gold was lost due to the lack of a smelting plant.

An extensive cyanide plant was constructed at King Cassilis mine about 1896, but was not a great success.

By the first world war, most of the available gold was depleted, and both mines closed down. The town rapidly declined and by 1933 there were only 34 inhabitants. The 1939 bushfires destroyed part of the town, and other buildings were taken away. Now Cassilis lies as a ghost town, with no commercial buildings in operation. Set in the 3600 hectare Cassilis Historical Area, the mines are well worth a visit, with many relics from the gold mining past.