Some One Piece Pirates have left their pirate life, and joined the Marines for some reason, instead. However, such pirates are very few in number.
Today, I have made a list of characters that were pirates but later they joined the Marines.
Jango was a member, and also first mate of Black Cat Pirates. When Kuro faked his own death, Jango became their second Captain. After the events of Syrup village, Kuro was captured by the Marines. Jango then joined the Marines and become an underling Rear Admiral Hina. He was also seen during the battle of Marineford.