King Vultures are monogamous and parents share incubation and brooding responsibilities (Ferguson-Lees and Christie 2001). Females reach sexual maturity earlier than males, typically around age four or five. King Vultures typically breed during the dry season (Houston 1994) although Ferguson-Lees and Christie (2001) report that nests can be occupied in almost any month and that eggs have been recorded from February – July and October - December. One study reported that incubation occurs in October and November in Brazil and the nestling developed from November to April (Mendes de Carvalho et al. 2004). Clutches consist of one white egg and the chick is hatched semi-altricial (Schlee 1994). Two nests in Panama (Smith 1970), two nests in Venezuela (Ramo and Busto 1988, Schlee 1995), one nest in Brazil (Mendes de Carvalho Filho et al. 2004) and a single nest in El Salvador (West 1988) have been observed. All nests held a single egg or a single nestling. One nest was a scrape on the forest floor near the base of a spiny palm, one in a natural cavity in a rotten tree stump about 0.3 m above the ground, one in a hole in the limb of a Pithecolobium saman tree 10.5 m above the ground (hole dimensions 1.2 m high x 60 cm wide) another in a cavity in a Terminalia oblonga, one at the back of a natural recess among logs and branches piled over the base of a felled tree at 30 cm high, and one in the crevice of a 70 m high limestone cliff. The eggs rested on the ground or the floor of the stump, cavity, or recess. Both nests were in dense, wet, lowland second growth forest in Panama; the nests in Venezuela in dry tropical or dry deciduous forest. The species has also been documented to nest within Mayan ruins (Martinez and Monica 2008, Martinez 2008). Eggs were described as white, cream-white, or dull dirty white and without spots; measurements of eggs were 90 x 63 mm and 93 x 64 mm (Smith 1970), 95.6 x 63.6 mm (Mendes de Carvalho Filho et al. 2004) and 102.5 mm x 63.6 mm (West 1988). Incubation in captivity is from 56-58 days (Brown and Amadon 1968) and Ferguson-Lees and Christie (2001) report incubation from 50-58 days.
At one nest Smith (1970) was able to confirm that both individuals of the pair incubated based on the primary feather molt pattern on one individual. One individual incubated and the other perched or soared nearby; in the presence of the observer the adults uttered low croaking noises, snapped their bills from a crouching position with wings held open (described as "arched forward in an owl-like manner"). On two occasions the pair of adults approached the nest observer hopping on the ground to within 3 m (Smith 1970). Similarly Ramo and Busto (1988) observed adults perched nearby, hopping from branch to branch in nest tree nervously, or circling overhead. During the first seven visits to the nest (Ramo and Busto 1988) the nestling responded to observers by vocalizing constantly and bowing head toward feet (described by Schlee (1995) as a hooked-neck alarm display characteristic of nestlings); if observer attempted to touch the nestling, it attacked with talons and beak. Later the nestling no longer bowed but remained erect. Mendes de Carvalho Filho et al. (2004) reported nestling response to observers to indicate intense stress including regurgitation motions and bowing, and by 1 month of age they displayed aggressive behavior in the form of stamping their feet and attacking with their beak. Captive nestlings were reported to begin straying away from the nest site at 30-35 days of age (Schlee 1994). Ferguson-Lees and Christie (2001) report fledging at 72-86 days. Mendes de Carvalho Filho et al. (2004) reported that the nestling abandoned the nest at approximately 130 days after hatching.