He has written as a freelancer for many outlets, including The New York Times, Washington Post, and online edition of The New Yorker.

The Birth of the Pill (2014), Eig's fourth book, told the story of the renegades who invented the first oral contraceptive.

[9] In a 2017 review of Ali: A Life, Joyce Carol Oates, writing for The New York Times, said: "This richly researched, sympathetic yet unsparing portrait of a controversial figure for whom the personal and the political dramatically fused could not come at a more appropriate time in our beleaguered American history….

Much in its pages will be familiar to those with some knowledge of boxing but even the familiar may be glimpsed from a new perspective in Eig's fluent prose; for pages in succession its narrative reads like a novel — a suspenseful novel with a cast of vivid characters who prevail through decades and who help to define the singular individual who was both a brilliantly innovative, incomparably charismatic heavyweight boxer and a public figure whose iconic significance shifted radically through the decades as in an unlikely fairy tale in which the most despised athlete in American history becomes, by the 21st century, the most beloved athlete in American history.

Reviewing the book for The New York Times, Dwight Garner stated that it "supplants David J. Garrow's 1986 biography Bearing the Cross as the definitive life of King".