Along with five other British webcaps, C. ainsworthii was selected by Kew Gardens as a highlight of taxa described by the organisation's staff and affiliates in 2020.

Cortinarius ainsworthii was described in a 2020 research note in the journal Fungal Diversity by Kare Liimatainen and Tuula Niskanen.

Based on this analysis, as well as morphological and ecological factors, Liimatainen and Niskanen concluded that C. hydrobivelus was a synonym of C. armeniacus, leaving their species (now described as C. ainsworthii) lacking a valid name.

In a year-end round-up, Kew scientists selected ten highlights, one of which was six newly described British Cortinarius species: C. ainsworthii described from Brighton; C. britannicus from Caithness; C. scoticus and C. aurae from the Black Wood of Rannoch; C. subsaniosus from Cumbria; and C. heatherae from Heathrow Airport.

[2][3][4] In a press release, Kew identified Cortinarius species as "ecologically important in supporting the growth of plants, particularly trees such as oak, beech, birch and pine" and playing "a key role in the carbon cycling of woodlands and providing nitrogen to trees".

The spores are moderately dextrinoid, meaning that they stain reddish to reddish-brown when treated with Melzer's reagent or Lugol's solution.

It can be distinguished from closely related species by the combination of brown cap; almond-shaped, medium-sized spores (averaging 8.7 by 5.3 μm), and its habitat in deciduous forests on calcareous ground.

[1] Cortinarius ainsworthii can be found in deciduous forests (perhaps associating with oaks, hazels, and beeches) on calcareous ground, also in open, grazed areas, presumably with rock roses.