The iconic outlaw Billy the Kid will not receive a posthumous pardon after all, New Mexico Gov. Bill Richardson announced Friday, his last day in office.

Mr. Richardson had been considering whether to give the Kid a pardon based on sketchy, but plausible, historical evidence that the gun-slinging, cattle-rustling, sheriff-shooting outlaw had been promised clemency by the territorial governor in the 1880s, Lew Wallace.

Historians...