He illustrated numerous magazines, children's books and paperbacks. His comic book debut was in 1990 with adaptations of The Tales of O. Henry and 20,000 Leagues Under the Sea for the Classics Illustrated series. He went on to work for Dark Horse Comics, where he contributed to Indiana Jones and the Shrine of the Sea Devil and The Shadow. After John Cullen Murphy retired from Prince Valiant in 2004, Gianni began drawing the strip, continuing until March 25, 2012, when Thomas Yeates became the strip's illustrator on April 1, 2012.
