After starting is criminal career as a Maggia hitman, the man now known as Hammerhead was beaten so badly in a fight that his skull was shattered. After finding him laying in an alleyway, Jonas Harrow replaced the young man's skull with one made of titanium, featuring a broad, flat top.
Adopting the persona of the the 1920s gangster from a movie poster, which was one of the only things he remembered prior to his fight, the young man branded himself Hammerhead, and began his climb up Manhattan's organized crime ladder.
Hammerhead is constantly in conflict with Spider-Man, and more often than not The Kingpin of Crime, in his pursuit of underworld power, and has even been ambitious enough in these pursuits to form his own army during the Civil War.
To match his gangster persona, Hammerhead can always be seen wearing a three-piece suit, smoking a cigar, and carrying his trusty Tommy Gun.