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He was corporation counsel for Queens County from 1893 to 1899, and was elected as a Democrat to the 56th United States Congress, holding office from March 4, 1899 to March 3, 1901.
In 1982, New York City Mayor, Ed Koch appointed deLeon senior assistant corporation counsel on the recommendation of Frederick A. O. Schwarz Jr., New York City’s corporation counsel.
He was sheriff's attorney in 1913 and corporation counsel for the city of Utica in 1914; in 1922 he was an unsuccessful candidate for election to the 68th United States Congress and in 1928 to the 71st United States Congress.
He was in private practice in Fort Fairfield, Maine from 1883 to 1888, and in Seattle, Washington from 1888 to 1909, serving as corporation counsel to Seattle from 1892 to 1894.