The man who called himself Prince Khalid al-Saud of Saudi Arabia liked the finer things in life: Rolex watches and Cartier bracelets, a Miami penthouse with a door buzzer labeled Sultan and expensive outfits for his Chihuahua, Foxy, whose life he chronicled on Instagram.

Sometimes he drove his Ferrari, equipped with diplomatic license plates, to fashionable department stores, where he spent thousands of dollars at a time, prosecutors said. But his charmed life came to a screeching halt in 2017 when he was arrested and charged with using a fake identity to defraud investors.

It turns out he was not a Saudi prince at all.

On Friday, the man, Anthony Gignac, 48, was sentenced to more than 18 years in prison for posing as a member of the Saudi royal family and pulling off one scam after another, culminating in a scheme to defraud dozens of investors out of more than $8 million — money that prosecutors said was used “to finance his lavish lifestyle.”

It was not the first time Mr. Gignac, a Colombian orphan adopted by a Michigan family in 1977, used the al-Saud name to get what he wanted.