The river is about 24 miles, or 39 kilometres, long. Its source lies in the mountains to the south of Tonypandy, near the town of Tonyrefail. The river's numerous sources rise in the eastern slopes of Mynydd Penygraig (Penygraig) and Mynydd y Gilfach (Penrhiwfer), and the western slopes of Mynydd Dinas (Williamstown), whose eastern slopes feed the Rhondda by Porth. The source of the Ogwr Fach lies just to the west.
The Ely flows through Tonyrefail, and then through the grounds of the Royal Glamorgan Hospital at Ynysmaerdy, following the main A4119 (Tonypandy to Cardiff Bay route through the valley pass formed between Mynydd Garthmaelwg (Llantrisant Forest), to the west and Llantrisant, to the east. Flowing through Talbot Green, the Ely is joined by the Afon Clun at Pontyclun, before heading east to Miskin. Although numerous smaller streams join the river, the Clun is its only large tributary.
The Ely turns southeasterly reaching the Vale of Glamorgan and wending its way alongside the rural South Wales Main Line immediately south of Miskin and being traversed by the M4 motorway it flows through farmland which as part of the flood plain the South Wales Railway used to engineer genteel gradients under Isambard Kingdom Brunel. It passes the Museum of Welsh Life at St. Fagans and then continues into City and County of Cardiff flowing through residential Cardiff suburbs such as Pentrebane and Caerau and Ely (Welsh: Trelái) before flowing under the Vale of Glamorgan Line and into Cardiff Bay, by Penarth Marina.

GAC fishing on River Ely

Starts in the first field as you leave Fairwater, Cardiff on Cardiff Road (single North bank fishing only) for a short distance until the River goes under a railway bridge (circa 150m). The River returns into the next field from underneath another railway bridge. From here we have double bank fishing through and past St Fagans, under another railway bridge, until you reach circa 50m before the A4232 dual carriage way (circa 2.2kms of double bank fishing). From here the fishing is single bank North bank fishing of river only. This single bank fishing continues under the A4232 all the way to a point where the Nant Dowlais enters the River Ely just north of the village of St Georges (circa 1.7kms).