Born near Oaktown, Indiana, Cullop attended the common schools. He was graduated from Hanover (Indiana) College in June 1878. Professor for two years in Vincennes (Indiana) University. He studied law. He was admitted to the bar in 1881 and commenced practice in Vincennes, Indiana. He served as prosecuting attorney of the twelfth judicial circuit 1883-1886. He served as member of the State house of representatives 1891-1893. He served as delegate to the Democratic National Conventions in 1892 and 1896.
Cullop was elected as a Democrat to the Sixty-first and to the three succeeding Congresses (March 4, 1909-March 3, 1917). He was an unsuccessful candidate for renomination in 1916. He was an unsuccessful candidate for the Democratic nomination as United States Senator in 1926. He resumed the practice of law and was also interested in various business enterprises. He died in Vincennes, Indiana, October 9, 1927. He was interred in Greenlawn Cemetery.