DeLeon retired as a Municipal Court Judge last year after 34 years on the bench.
He said that gave him a unique view on what he calls a traumatized city, and how to help heal it.
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"I know that I can make a positive difference in our city and lead it to new heights. I believe that my experience, values, and vision would prove invaluable to the position of mayor. With your help and support, we could help make Philadelphia a better place," he said in a statement.
DeLeon joins six other Democrats who've already declared their candidacy for mayor.