It was first described in the literature in 1907 by Charles Horton Peck, based on specimens found by A.M. Patterson, a botany student at Stanford University.

[5] The cap, 5–19 cm (2.0–7.5 in) in diameter, is initially almost hemispherical in shape, transforming to broadly convex and finally to flattened or with edges upturned in age.

The cap flesh is typically 1–3 cm (0.4–1.2 in) thick, firm, white, and stains deep red 20–30 seconds after injury or bruising.

The light-cinnamon color in young specimens turns to a dark blackish brown in age after the spores develop.

[4] Agaricus pattersoniae is associated with cypress (Cupressus macrocarpa) trees, where it fruits singly or in groups.

Richard Kerrigan noted that it seems to be restricted to mature trees (age 40+ years) located in the coastal counties of California,[4] although Alexander H. Smith mentions that it has also been found in the Great Lakes regions.