Allen was an attorney from Maine and arrived in Texas in 1839, establishing a law practice in Clarksville. He was appointed as the Texas Attorney General under President Anson Jones and helped with the process of annexation. He moved to Galveston in 1846 and was the first elected AG in Texas in 1850. Allen also secured a charter for a railroad and served as a promoter for the Houston & Texas Central Railroad. The North Texas town of Allen is named after him when the Houston & Texas Central Railroad built a depot there.