Is Regan a first name or a last name?

Regan is both: a family name in some places and a given name in others. It is mostly a family name in the United States, Canada and Ireland.

How to address someone called Regan

United States. Regan is a family name, usually written last. Write Dear Mr. Regan or Dear Ms. Regan.
United Kingdom. Regan is used both ways: 76% of people from the United Kingdom with this name in our data carry it as a family name. Look at where it sits in the name and how the person signs.
Canada. Regan is a family name, usually written last. Write Dear Mr. Regan or Dear Ms. Regan.
Ireland. Regan is a family name, usually written last. Write Dear Mr. Regan or Dear Ms. Regan.

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 Regan as a family name or as a given name, by country of citizenship (151 in total).

Country	As family name	As given name	Family name share
United States	50	12	81%
United Kingdom	26	8	76%
Canada	17	4	81%
Ireland	13	0	100%
Australia	5	2	71%
New Zealand	0	4	0%

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

As a surname: about 21,951 people in the 2010 census, rank 1,642 among US surnames.
As a first name: 18,806 babies born between 1950 and 2017, 82% of them girls.