The longfin mako has a dark bluish back and white underside. It appears to be the larger of the two mako species, with a average adult length of 3.7 m (12.3 ft) and mass of 343 kg (756 lb). The maximum known size is 4.27 m (14 ft). The pectoral fins are about as long as the head or longer, relatively broad-tipped in young and adults. The snout is usually narrowly to bluntly pointed, usually not acute. The cusps of upper and lower anterior teeth are straight, with tips not reversed. The caudal fin is lunate, with a very long lower lobe.
The longfin mako shark is a yolk-sac ovoviviparous shark, meaning it gives birth to live young which feed from a sac full of yolk in the womb. The gestation period for a mako shark is 15–18 months. Embryos in the female's body consume each other to get nutrients, with only two pups usually produced in a litter. This intrauterine cannibalism is common in sharks.
This species is often of slimmer build, and its long, broad-tipped pectoral fins suggest that it is slower and less active, than its better-known relative the shortfin mako, Isurus oxyrinchus.