Mildred combines the Old English mild (gentle) and þryð (strength) — meaning "gentle strength." Saint Mildred of Thanet was a 7th-century Anglo-Saxon princess and abbess.
Mildred peaked at #6 in the U.S. in 1912. Mildred Loving (1939-2008) was the plaintiff in Loving v. Virginia, the landmark civil rights case.
Mildred does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Mildred reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.