Genicanthus lamarck adults are pale greyish to whitish in colour marked with 4-5 irregular black undulating stripes which radiate out from the eye and run horizontally along the flanks. There is a wide black submarginal band on the dorsal fin, and the caudal fin is finely spotted. The topmost stripe is wider in the females and in the juveniles. The males have a yellow patch on the crown, black pelvic fins, and very elongated lobes on the caudal fin. In addition the males have black pelvic fins and white lobes to the caudal fin while females have white pectoral fins and black caudal fin lobes. The dorsal fin contains 15 spines and 15-16 soft rays while the anal fin has 3 spines and 16-17soft rays. This species attains a maximum total length of.
Genicanthus lamarck is found at depths between. It is found in areas of dense coral growth on the seaward slopes of reefs and on steep drop-offs. Its normal diet is zooplankton. They are normally observed over the bottom in small harems with a dominant male and 2-6 females. They are sequential protogynous hermaphrodites and if the male in a harem goes missing the dominant femalechanges sex to become male.