At some point, every writer has to submit a short paragraph about themselves along with their query letter, article, or short story. It’s a good idea to prepare several of different lengths and content as the preferred style varies with the type of material you are submitting.
For a query letter, write in first person. Keep it short, preferably under 50 words, and relevant to the specific work you are pitching. Mention any previously published work, education, or experience only if it is related to what you are querying. (Unrelated education or experience may be useful when a longer bio is needed, such as for a pamphlet or book jacket.) If you have relevant writing credits, list the best three; select the most prestigious, and of those choose the most recent.
If you don’t have credits related to what you are querying, mention something specifically connected to the article, story, or book that you are pitching. For example, you might tell what gave you the idea for it, or how you became interested in that subject. Stick to the point; generalities about being a life-long dream, loving children, etc. don’t really explain why you are the best person to write whatever it is that you are trying to sell.
For bios that accompany an article, write in third person, and make it interesting. Usually editors will list the approximate number of words to include for the bio, but if they don’t, keep it under 25 words. This is really for your readers, so mention something fun or memorable. It’s not possible to list all your credits, but you might list your website, where all the additional information about you should be available.
Here are a few sites with additional suggestions for writing your author’s bio:
EDIT 7/6/11: Agent Rachelle Gardner has a helpful post on this topic on her blog today.
Edited to add: To see an example of an author bio for an article, see the post dated April 4, 2009, Tax Deadline Approaches. Note that the bio is written in third person, contains credentials, and also includes a bit of personal info that gives the reader a sense of who the author is. It also provides a link to the author’s own website, giving readers a way to learn more about him.