An estimated 900 fans went on the rampage outside the club's New Den ground in south-east London after the division one play-off between Millwall and Birmingham City on 2 May.
One police officer, who had been involved in the May Day and Poll Tax riots, said it was the worst violence he had seen.
Metropolitan Police officers have been impressed with action taken since the unrest and by the club's donation to a charity helping injured officers.
Charitable donation
Millwall has introduced several steps to combat hooliganism - including banning fans from six Division One clubs from going to their ground.
A Scotland Yard spokesman said: "The Metropolitan Police Service has been impressed with the steps taken so far by the club, and its investment in technology, which has dramatically reduced problems and anti-social behaviour.
"In light of the efforts made and a donation to a charity helping injured police officers, the Metropolitan Police Service has decided not to pursue legal action against Millwall FC in relation to the disorder."
Police say they have made only three football-related arrests of Millwall supporters this season.
Some legal experts believe it would have been difficult to hold a football club responsible for something that occurred outside its ground which may have involved people who did not attend the match.
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