George Washington is the highest ranking officer to have served in the United States military, but it took 178 years since his last promotion and 177 years after his death before that accomplishment was realized.
Washington rose to the rank of lieutenant general in 1798, by order of President John Adams, in preparation for an anticipated war with France. While that war did not materialize, Washington remained as the senior military officer in the United States until his death seventeen months later.
On September 3, 1919, General John J. Pershing was promoted to the rank of General of the Armies of the United States. He was the only person to be awarded this rank while still living. In 1976 Congress declared that no one should outrank George Washington, so they authorized his promotion, giving him seniority over Pershing, who, by this point, was also deceased.