the league was established in december 1907. lawson had founded an outlaw baseball league before the 1907 season; he called it the atlantic league, a name also used by multiple other eastern leagues in baseball history. lawson took three atlantic league teams with him when he founded the union professional league. the final list of teams included clubs from philadelphia, pennsylvania; newark, new jersey; elizabeth, new jersey; paterson, new jersey; brooklyn, new york; washington, dc; wilmington, delaware; reading, pennsylvania and baltimore, maryland.