Mr. Brown migrated to Pumkin World after his parents tried to flee the Police to escape paying taxes. He grew up in a 20ft by 20ft box. As a child, he got made fun of because he couldn't understand English. The other children would pick on him and say "do you want a $100!" and he would say the only word he knew, "No!" He failed all of his classes but still managed to go to college. In college, he sold drugs and started a gang called "El Pumkin Seeds." He is still considered the leader, even though he doesn't take an active role I the gangs operations any more.
Mr. Brown is a badass Pumkin who moved to Bikini Bottom to grow the "El Pumkin Seeds" gang. He doesn't take crap from anybody because he kills everyone who a mouths him.