His written works include religious and political essays, humor, adult fiction, and juvenile fiction, and he is best known for his 1974 book Selma 1965: The March That Changed the South, his in-depth history of the 1965 Selma Voting Rights Movement, which led to the passage of the Voting Rights Act.

From 2002 to 2012 Fager served as Director of Quaker House in Fayetteville, North Carolina, a peace project founded in 1969 near Fort Bragg, a major US Army base.

There he was in the Air Force Reserve Officers' Training Corps at Colorado State University, where he won a medal as the Outstanding Freshman Cadet, and later commanded a prize-winning AFROTC drill team.

[8] In the late 1970s, Fager was briefly active in the anti-abortion movement, making connections with some anti-war-minded activists in it.

However, as the political and religious right essentially absorbed the anti-abortion movement in the early 1980s, Fager moved away from it, repudiated its increasingly rightwing and repressive character, and also reconsidered his understanding of the embryology and metaphysics involved.

Fager first met Quakers in Selma, Alabama in late 1965 when students from the newly launched Friends World Institute came to help with voter registration.

He began work as a journalist in college, and in 1967, published his first book, White Reflections On Black Power, followed in 1969 by Uncertain Resurrection: The Poor Peoples Washington Campaign.

In the early 1970s he was commissioned by Charles Scribner's Sons to write a history of the Selma Voting Rights Movement.

By 1978, after Fager had moved to the Washington DC area, McCloskey hired him as a Congressional staffer for the U.S. House Merchant Marine and Fisheries Committee.

Despite his high regard for McCloskey, Fager was not drawn to stay on Capitol Hill, and was content to leave it shortly thereafter for the renewed uncertainties of the freelance writer's path.

Beginning in 1981, he also edited an independent, muckraking and gadfly Quaker newsletter called A Friendly Letter, which continued until early 1993.

After leaving the Postal Service in 1994, Fager was hired to create an Issues Program at Pendle Hill, a Quaker study center in Wallingford, Pennsylvania.

In 2013–2014, he was the Cadbury Scholar in Quaker history at Pendle Hill; while there he researched and wrote two books on the "Progressive Friends" movement of the 19th and early 20th centuries.

[11] In his writing, Fager has pursued several abiding interests: reporting, especially about current social issues such as the Civil Rights Movement, recent wars, militarism, and torture; religion, with special focus on Quakerism, or the Society of Friends; and stories, particularly for younger readers.

In July 2013, Fager was arrested in a peaceful protest that was part of the "Moral Mondays" campaign in North Carolina.