Maxwell was born in a LOOOOONG time ago, after appearance of planets. He was the 14th guy in UnUniverse. Using nuclear reactions and dark matter he created a book and a pencil. He uses it to write stuff and materialize it.
During most of time he was outside of UnGalaxy, working in a galaxy far far away, but he sometimes appeared in UnWorld. He created Stonehenge when he wrote "Nightclub", Taj Mahal ("Hotel"), the Leaning Tower of Pisa ("Tower"), Disney World ("Small park with a house").
He has a big army known as "Scribblenauts Army". Usually it's a peacekeeping army, but sometimes Maxwell uses it in violent wars. For example he stated genocide of Jupiter's population who almost realized how Maxwell uses his book.