The tortoise's latest Guinness World Records title is officially "oldest chelonian," a category including all turtles, terrapins and tortoises.
The previous record-holder was Tu'i Malila, a radiated tortoise that died at the estimated age of 188 in 1965.
The St. Helena government said Jonathan is blind and has lost his sense of smell, but eats well when hand-fed by his veterinary team.
Veterinarian Joe Hollins said Jonathan still is active, and often enjoys the company of fellow giant tortoises David, Emma and Fred.
"In spite of his age, Jonathan still has good libido and is seen frequently to mate with Emma and sometimes Fred -- animals are often not particularly gender-sensitive," Hollins told Guinness.