Joseph Rowntree was pioneer in north Etobicoke village of Thistletown and established two mills on the banks of the Humber River. In 1843 he built a sawmill on the east bank of the river. Five years later, he built a grist mill on the west bank. His mills were known as the "Greenholme Mills" and operated until the end of the nineteenth century.

The park was named Rowntree Mills Park in 1969 in honour of Rowntree.

The Humber River is one of two major rivers on either side of Toronto, Ontario, Canada, the other being the Don River to the east. It was designated a Canadian Heritage River on September 24, 1999.

The Humber collects from about 750 creeks and tributaries in a fan-shaped area north of the city. One main branch runs for about 100 km from the Niagara Escarpment to the northwest, while the other major branch starts in the Lake St. George in the Oak Ridges Moraine near Aurora, Ontario to the northeast. They join north of Toronto and then flow in a generally southeasterly direction into Lake Ontario at what was once the far western portions of the city.

Unlike the Don to the east, the Humber remained relatively free from industrialization as Toronto grew, mainly because it is much flatter and does not provide a large river valley to build in. Since Hurricane Hazel showed the land to be unsuitable for housing, it has been largely developed or redeveloped as parkland, with the extensive and important wetlands on its southern end remaining unmolested. Whereas the mouth of the Don is often clogged with flotsam and is obstructed by low bridges, the Humber is navigable and a major sporting and fishing area.

Today the majority of the Toronto portion of the Humber is parkland, with paved trails running from the lakeshore all the way to the northern border of the city some 30 km away. Trails following the various branches of the river form some 50 km of bicycling trails, much of which are in decent condition. Similar trails on the Don tend to be narrower and in somewhat worse condition, but the complete set of trails is connected along the lakeshore, for some 100 km of off-road paved trails.