Little is known about the reproduction of king mackerel. Spawning occurs most frequently during May through September. Eggs are believed to be released and fertilized continuously during these months, with a peak between late May and early July with another between late July and early August. The ovaries of the king mackerel have five stages of development. Maturity may first occur when the females are as small as 17.7-19.6 inches (450-499 mm) in length and usually occurs by the time they are 35.4 inches (800 mm) in length. Stage five ovaries, which are the most mature, are found in females by about age 4 years. Males are usually sexually mature at age 3, at a length of 28.3 inches (718 mm). Females in U.S. waters, between the sizes of 17.6-58.6 inches (446-1,489 mm) released 69,000-12,200,000 eggs. Because both the Atlantic and Gulf populations spawn while in the northernmost part of their ranges, there is some thought that they are reproductively isolated groups.
Larvae of the king mackerel have been found in waters with temperatures between 79-88°F (26-31°C). This stage of development does not last very long. Larva of the king mackerel can grow up to 0.02 to 0.05 inches (0.54-1.33 mm) per day. This shortened larval stage decreases the vulnerability of the larva, and is related to the increased metabolism of this fast-swimming species. The larvae can be identified by their large, wide open jaws, well-developed teeth, large eyes, and a pointed snout. The coloration of the brain develops early and retains the color throughout life. The color of the trunk develops at about 0.8 inches (20 mm). The fins remain unpigmented until the fish reaches 1.2 inches (31 mm) in length, with the caudal fin remaining unpigmented.