Body shape is extremely elongated and strongly compressed laterally. Color distinguishes it from its relatives. It lacks the wavy vertical lines found on the body of the wahoo and the pronounced black spot on the front of the dorsal fin in the Spanish mackerel and the cero. Body color is silvery-gray to iridescent green. Young king mackerel have spots on their sides similar to Spanish mackerel, but can be differentiated from the latter fish by having a sharper bend in the lateral line midways down the side.

King mackerel are considered to be migratory fish, but a resident population of large fish (commonly over 40 pounds) remains year round in the northern Gulf of Mexico, off of Louisiana. King mackerel in the eastern Gulf winter off of Florida and in the summer migrate up the Florida coast to the northern Gulf of Mexico, with some moving as far as Texas. Western Gulf king mackerel move between Louisiana, Texas and Mexico, spending their winter months in the south and summer in the north. Waters off of Louisiana serve as a mixing ground for eastern Gulf, western Gulf and large resident fish. King mackerels can be found alone, but are usually in small groups.

King mackerel are aggressive, hard-hitting predators that specialize in feeding on other fishes. By far the most common fishes eaten are herrings, including menhaden and sardines. These are followed by seatrout, primarily sand and silver seatrout, and then various jacks. Other species are also eaten. Researchers have found that most fish eaten by king mackerel are less than 8 inches long, generally averaging 4-6 inches. Surprisingly, the average size of fish eaten did not increase in larger king mackerel. Non-fish food items make up only a small amount of the diet, with shrimp being the leading category. King mackerel can live to at least 14 years, although most die earlier. Females grow larger than males and spawn in their third or fourth year of life. Spawning takes place in the warmer months.