Adult mourning cloaks are maroon-brown above with yellow borders. A row of small blue spots run along the inside of the yellow border. Undersides of wings have cream-colored borders on a dark, striated (finely lined) grayish or blackish background. Outer edges of wings are irregular, with a few small projections.
The larvae, sometimes called spiny elm caterpillars, live and feed communally in webs in their host trees. They can reach 2 inches in length and are black with tiny white dots. They have black spines and a line of reddish spots run down the back. The legs are dark red.