One who does something: a doer of evil deeds. (noun)
A particularly active, energetic person: a real doer in party politics. (noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "doer" in a sentence
"Doubtless Jelchs will make outcry when the hand of the evil-doer is nigh him."
"A non-doer is very often a critic-that is, someone who sits back and watches doers, and then waxes philosophically about how the doers are doing."
"Whatever normative analysis they apply when the doer is a private entity doesn't seem to apply when the doer is a government."