This was the most successful period in the club's history. The first full season after World War I the club were relegated to the new Third Division; in the initial 1920–21 season they played against the former members of the Southern League who had been invited to form the new division, but after a year an equivalent Third Division North was created and Grimsby moved across to that. By 1929 they were back in Division One, where they stayed (with a brief break from 1932 to 1934) until 1939, obtaining their highest-ever league position, 5th in Division One, in the 1934–35 season. In 1925 they adopted the black and white stripes as their colours.