The Long-billed Crombec is a small, nearly tailless bird 12 cm long and weighing around 16 g. Its upperparts are brownish grey-brown, and there is a pale grey supercilium, separated from the whitish throat by a dark eye stripe. The whitish breast shades into the buff belly. The long slightly curved bill is blackish.
The sexes are similar, and the juvenile resembles the adult. The call is a variable series of trilled notes including trreee-rriiit trreee-rriiit and a harsh pttt.