Senecio inornatus is a robust perennial herb with one or more stems from an underground rootstock, reaching up to 2m high. Plants are usually hairless but can also be covered by a white cobwebby woolly layer. Most of the leaves are basal and can be up to 500×35mm, diminishing in size upwards along the stem, generally sessile and narrowing to both ends. The leaf blade often extends somewhat along the stem at the base. Leaf margins are dentate, but usually only have small wart-like outgrowths along margins.

The compound inflorescences are flat-topped, with numerous small flowerheads. Involucral bracts (bracts surrounding the base of the flowerhead) consist of about 10-12 in a single row, with a few, much shorter calyculus bracts (the bracts around the involucre). Capitula (dense head-like inflorescences) are radiate with 5-8 ray florets. Florets are yellow. Flowering time is from December to March.