Gianni graduated from the Chicago Academy of Fine Arts in 1976. After graduation he worked for the Chicago Tribune as an illustrator and Network Television News as a courtroom sketch artist.
He also illustrated numerous magazines, children's books and paperbacks and made his comic book debut in 1990, with illustrated 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, contributing such titles as Indiana Jones and The Shrine Of The Sea Devil and The Shadow.
He won the 1997 “Best Short Story” Eisner Award for his collaborating with Archie Goodwin on Heroes in DC Comics Black & White.
He illustrated Wandering Star Press’s Savage Tales of Solomon Kane (1998) and Bran Mak Morn: The Last King (2001) by Robert E. Howard.
Gianni took over the newspaper comic Prince Valiant after the retirement of John Cullen Murphy in 2004.