He graduated from the law department of Columbia College in 1828, was admitted to the bar, and commenced practice in New York City.
McKeon was a representative in the New York State Assembly from 1832 to 1834. He was elected as a Jacksonian to the 24th United States Congress, serving from March 4, 1835, to March 3, 1837, but was defeated for re-election. He was elected as a Democrat to the 27th United States Congress, serving from March 4, 1841, to March 3, 1843, but was again defeated for re-election.
In February 1846, he was appointed New York County District Attorney and, when the office became elective under the State Constitution of 1846, was elected in May 1847 to succeed himself.
He remained in office until the end of 1850 when his term expired. In this office, he secured the conviction of Madame Restell.
He was appointed by President Franklin Pierce United States Attorney for the Southern District of New York, and served from July 10, 1854, to January 7, 1858. While holding this office, he prosecuted a number of important cases.
Among them were the attempt to enlist men to serve in the British Army during the Crimean War, and the seizure of the filibustering ship “Northern Light.”
He was again New York County Doctorate.A. from 1882 until his death in office.
He died at his residence at 44, West 37th Street, and was buried in a family vault under the old Saint Patrick"s Cathedral on Mott Street in New York City.
Member of New York Assembly, 1832-1834. Member United States House