"Like me, Meghan probably never dreamt that she'd have a life like this", Obama said in a new interview for Good Housekeeping's January issue.
She said: "So my biggest piece of advice would be to take some time and don't be in a hurry to do anything".
As the crowd exploded in response to Obama dropping a curse word, she laughed, "I forgot where I was for a moment!".
It was an unusual critique of Mrs Sandberg, who wrote a book entitled Lean In in 2013 that encourages women to get ahead in the workplace by "leaning in" to their ambitions.
Obama is promoting her new book, Becoming, which sold two million copies in the United States and Canada in just 15 days.
Michelle Obama enjoyed an emotional return to a north London secondary school she credits with inspiring her to get involved in educational reform. "It is hard. I love my husband, and we have a great marriage, and we've had a great marriage, but marriage is hard work". As writer and cultural critic Touré tweeted, "The substance of that sentence - a rejection of Sheryl Sandberg's controversial ethos - is much more interesting".
In a study this year, a group of Duke University psychology professors used 2,000 participants to test the "lean in" approach and how it worked with structural and systemic disadvantages women face in the workplace.
"Despite waiting in the rain for an hour to get into the Barclays Center, I was just in the same room as @MichelleObama and she accidentally said "shit" and it was awesome and my life has been forever changed", one user wrote. "I think it's okay - it's good, even - to do that...."
"In terms of women in leadership roles, we are not better off", she said, noting that women occupied less than 6 percent of Fortune 500 CEO jobs and their global equivalents, and the number of countries run by women had decreased.
As she spoke with her friend and poet Elizabeth Alexander, Obama talked about the challenges of balancing career and family.
However, the unbelievable Michelle Obama has given Meghan some pretty incredible advice.