Is Lacy a first name or a last name?

Lacy is both: a family name in some places and a given name in others. It is mostly a family name in the United Kingdom.

How to address someone called Lacy

United States. Lacy is used both ways: about 23,110 people had it as a surname in the 2010 census, and 20,805 babies born between 1950 and 2017 were given it as a first name. Look at where it sits in the name and how the person signs.
United Kingdom. Lacy is a family name, usually written last. Write Dear Mr. Lacy or Dear Ms. Lacy.

Not sure which part of a full name is which? Check the full name with the country where the person is based.

People recorded in Wikidata who carry Lacy as a family name or as a given name, by country of citizenship (104 in total).

Country	As family name	As given name	Family name share
United States	51	26	66%
United Kingdom	8	2	80%
Australia	4	0	100%
Spain	3	0	100%

Wikidata covers notable people, so counts show how the name is used, not how many people carry it.

As a surname: about 23,110 people in the 2010 census, rank 1,559 among US surnames.
As a first name: 20,805 babies born between 1950 and 2017, 89% of them girls.