Slender wood sedge is a graminoid with a terminal, prominently peduncled staminate spike, a pistillate spike that is erect to drooping, the lowest spike basal, 6-18mm long. It has 3-9 uninflated, glaborous, three-sided perigynia per spike that are obovoid and with a short beak. Pistillate scales are acute. Leaves are erect or ascending, flat, 8-44mm long and up to 5mm wide.
It can be distinguished from the similar C laxiculmis by its narrower leaf blades.