He was unmarried and without children at the time of his death, meaning his sole heir was his sister. Millen was buried in that city's Laurel Grove Cemetery. There is a cenotaph for him at Congressional Cemetery in Washington, D.C.
He was single and childless at the time of his passing, making his only beneficiary his sister. Millen was interred in that city's Laurel Grove Cemetery. A monument dedicated to him exists at Congressional Cemetery in Washington, D.C.