Mill no. 1, known as "the big water mill," was built in 1865 by Daniel McLachlin on the East bank of the Madawaska river. This was not McLachlin's very first mill, however. In the 1850s, McLachlin was operating a mill he refurbished from the remnants of a previous mill built by the Buchanan brothers in 1833. It was water-powered and had a single saw which turned out one board at a time. When the Brockville & Ottawa railway was built to Arnprior in 1865, the new mill was constructed on the same site to produce much more lumber for transport to new markets.
With the building of the newer steam-saw Mills 3 and 4 in subsequent decades, the equipment in Mill No.1 slowly became outdated. In 1892, with Mill No.4 having just been built, it was deemed expedient to discontinue the work in Mill No.1. By 1905, the insurance plan for the McLachlin Bros. Sawmills & Lumber Yards lists Mill No.1 as a "late mill" repurposed as a storehouse.