Morgan was born in Washington, D. C. A 1987 graduate of Harvard College with a B.A. in English, Steiner's first published work was an autobiographical account of her teenage struggle with anorexia nervosa, published in Seventeen in September 1986. The article, "Starving for Perfection", was written under the pseudonym Isabel Johnson and received over 4000 reader letters, at the time a record for Seventeen, and appeared in the anthology The College Reader (Harper Collins, 1993).

Steiner went on to work in the Articles Department for Seventeen from 1987-1988. She was a freelance magazine writer and consultant from 1988-1990. She earned an MBA degree in Marketing from the Wharton School of Business at the University of Pennsylvania in 1992.

Her corporate marketing career included stints at the Leo Burnett advertising agency in Chicago and Johnson & Johnson in New Brunswick, New Jersey. At Johnson & Johnson she launched the low-calorie sweetener ingredient sucralose, also known as Splenda Brand Sweetener, internationally from 1994-2000. She oversaw the public relations program for the sweetener's United States Food and Drug Administration approval on April 1, 1999.

In early 2001 Steiner returned to her hometown of Washington, D.C. to become general manager of The Washington Post Magazine. While working for The Washington Post, Steiner became interested in the struggles of and tensions between American working and stay-at-home mothers. Her anthology, Mommy Wars: Stay-at-Home and Career Moms Face off on their Choices, Their Lives, Their Families was published in 1986 by Random House and the essays by a range of at-home and working mothers such as Jane Smiley, Susan Cheever, Carolyn Hax and Jane Juska generated extensive media interest and controversy among conflicted American mothers, including mommy bloggers, daddy bloggers and publications such as The Atlantic Monthly, The Los Angeles Times and elsewhere. Steiner continues to interpret the mommy wars, most recently the controversy created by the nomination of Alaska governor Sarah Palin as the Republican Party's vice presidential candidate, and Michelle Obama's position as the first African American first lady.

From 2006 to 2008, Steiner wrote On Balance, an online column exploring work/family conflicts for washingtonpost.com, the internet site for her employer. On Balance, one of washingtonpost.com's first forays into the blogosphere, quickly became popular among a diverse audience of men and women with and without children. Over the course of two years, Steiner's column became one of the most popular "mommy blogs" on the web. Steiner wrote over 500 columns, and the site accumulated over 100,000 comments from online posters. The readership weighed in with up to 700 comments per day. Over the course of 27 months, 112,898 total comments were submitted by 11,735 different posting names and 16,632 anonymous posters.

Steiner currently writes a weekly column, "Two Cents on Modern Motherhood", for Mommy Track'd. She is a contributor to On Being Fearless by Arianna Huffington and The Huffington Post. She recently finished Crazy Love, a memoir about surviving domestic violence that spent three weeks on the New York Times Bestseller List. Steiner is currently working on The Crazy Love Project, a documentary profiling abuse survivors.

Steiner is married to Perry Winter Steiner, a fellow Wharton business school graduate who works in private equity for Washington, DC - based Arlington Capital Partners. The couple has three children and lives in the District of Columbia.