The longest-serving United States Senator is Robert C. Byrd, Democrat of West Virginia.  Senator Byrd took his first oath of office over 50 years ago, on January 3, 1959, and was re-elected to an unprecedented ninth consecutive term in 2006.

Bonus Question:

Who was the oldest man ever to serve in the U.S. Senate?

Answer:

The oldest man ever to serve in the U.S. Senate, Strom Thurmond was 52 years old when he first arrived in the Senate in 1954 and 100 years old when he stepped down in 2003.

Source: U.S. Senate website