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Adults are of a uniform, drab colour. Head brownish, antennae brown, labial palpi porrect, heavily scaled, brown. Thorax brownish, metathorax with some rusty-coloured scales. Forewings with pointed apices. Dorsal surface of forewings dirty brown with various pale markings and spots. Discal region with an oblique, cream-coloured line that is slightly curved. Hindwings are of a uniform light grey. Ventral wing surfaces, legs and abdomen light grey. It can be distinguished from other microlepidoptera by its slightly hooked forewings, broad hindwings and porrect labial palpi. In Alberta this species can be confused with a pale morph of Ypsolopha flavistrigella, however the latter differs from Y. senex by a uniform grey colouring of forewings and absence of the whitish oblique line in the discal region. It can also be confused with Y. dorsimaculella, however dorsimaculella has two well-defined dark-brown spots on the internal margin of forewings, no hooked forewings and no whitish, oblique marking in the discal region of forewings.

Larva uniformly green with no distinct markings, head capsule green. Feeding occurs in a loose web. Like for most North American Ypsolopha species, the life cycle is not well understood, but it most likely has one generation per year (Ives & Wong, 1988). Pupation takes place in silken cocoons of a tubular shape.

A very common species occurring throughout North America. In Canada it occurs in most provinces, from B.C. (Scudder and Cannings 2007) and Alberta (Bowman 1951) to Manitoba (Ives & Wong 1988). Since willows are widespread in North America including the Canadian subarctic (Begin & Payette 1991) it is very likely Y. senex has a much larger distribution than currently known. This species is present in most of the continental U.S.