Knill is recorded in the Domesday Book (1086) as Chenille (and its name is believed to come from the Old English cnyll ("hillock"). There is indeed a small hill above the village, on which is Burfa Camp, a hill fort overlooking the brook and the gap it makes in the hills.
The population of the village in the 1861 census was 84 and this has now declined to figures in the 20s.
The parish church is St Michael and All Angels.
Knill is one of the Thankful Villages; those few to whom all the men returned alive after the First World War. It is one of three such villages in Herefordshire. Thus there is no war memorial in the village but in the church there is a carved stone plaque on the wall, which greets visitors with the inscription:
TO THE GLORY OF GOD
AND IN THANKSGIVING TO HIM FOR
THE VICTORY GRANTED TO US IN THE
GREAT WAR OF 1914-1918
AND FOR HIS MERCIES IN BRINGING
SAFELY HOME THE MEN OF THIS PARISH
WHO SERVED THEIR COUNTRY OVERSEAS
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