This is a list of all seasons played by Millwall Football Club from their early beginnings in the Southern League, to their inaugural season in 1920–21 in the English Football League and up to their last completed season. It details their record in the FA Cup, the League Cup and other major competitions entered, as well as managers, top goalscorers and average home attendance for each season. Millwall were founded in 1885 and for the first nine years of their existence did not compete in league football. They first entered the FA Cup in 1887, turning full-time professional as a club in 1890. They were founding members of the Southern League in 1894, which they competed in for 22 seasons, claiming the title twice. They left to join the Football League in 1920. Millwall have played in all four divisions during their 91 seasons in the league, including the Third Division South, which they won twice, in 1928 and 1938. Millwall were Fourth Division champions in 1962.
The 1988–89 season stands as The Lions most successful when Millwall reached the top tier of English football, where they finished 10th in their first ever season in the First Division, after being promoted as champions of the Second Division in 1988. Their lowest league finish is 9th in the founding season of the Fourth Division in 1958–59. Millwall were Second Division champions in 2001, where they finished with their highest ever points total of 93. They reached the 2004 FA Cup Final, losing to Manchester United, but still qualified for the UEFA Cup the following season, playing in Europe for the first time in their history. They've also reached the FA Cup semi-finals on four separate occasions, in 1900, 1903, 1937 and 2013. Millwall have also reached the quarter-finals of the League Cup on three separate occasions, in 1974, 1977 and 1995. Millwall won the Football League Group Cup in 1983 and were runners up in the Football League Trophy in 1999. They've reached the Football League play-offs eight times. They were play-off semi-finalists in 1991, 1994, 2000 and 2002, finalists in 2009 and 2016 and they were promoted as winners, in 2010 and 2017.