After earning his bachelor's degree, he studied for a year as a Fulbright Scholar at the Econometric Institute in Rotterdam.

At Berkeley, his researching and writing concerned family law, foster care, child custody, and other children's rights topics.

[2][8] After serving as a visiting professor for the 1990–91 academic year Mnookin joined the Harvard Law School faculty permanently in 1993 where he is the Samuel Williston Professor of Law, the Chair of the Program on Negotiation, and the Director of the Harvard Negotiation Research Project.

[1][2] Over the course of his career, Mnookin has served as a neutral arbitrator or mediator in a number of complex commercial disputes.

Along with co-arbitrator, John L. Jones, Mnookin helped resolve a software rights dispute between IBM and Fujitsu in 1987.

[2] The couple has two daughters:[8] Jennifer Mnookin, the Chancellor of University of Wisconsin Madison, and Allison, formerly a vice president and General Manager at Intuit.