Harris published New Attitude, a magazine aimed at fellow homeschoolers, from 1994 to 1997. His first book, I Kissed Dating Goodbye was published in 1997 and has sold nearly one million copies worldwide. Subsequent publications include Boy Meets Girl (2000), in part describing his engagement to his wife, Shannon; Not Even a Hint: Guarding Your Heart Against Lust, released in 2003 and renamed Sex Is Not the Problem (Lust Is) in 2005; and Stop Dating the Church!: Fall in Love with the Family of God (2004).

Harris' new book, which highlights his passion for what he calls "Humble Orthodoxy", is entitled Dug Down Deep and was released in early 2010. In it, Harris shares the importance for theology, his own journey towards a love for theology, and various theological concepts that changed his life.

Harris started and led Sovereign Grace Ministries' New Attitude Conference for Christian singles from 1999 until 2004. There was no conference in 2005, and New Attitude was revived in 2006 under the leadership of Covenant Life Church singles pastor, Eric Simmons. The conference took place over Memorial Day weekend, at the Louisville, Kentucky Convention Center through 2008. Josh then resumed leadership in 2009 over the conference and the conference was moved to the Baltimore convention center, and renamed Next. The 2011 conference will be held in Orlando, Florida.

The primary focus of New Attitude is to equip young Christians with what organizers believe is the biblical truth of the gospel, and to stress the importance of the church in the lives of all Christians. New Attitude's focus is that the truth should be conveyed with humility and as such the leaders of New Attitude have coined the term Humble Orthodoxy to represent these principles. New Attitude's website describes "Humble Orthodoxy" as "a commitment to believing, living, and representing the truth with humility. We believe that God's truth in Scripture should not be redefined or reinvented to suit our own preferences or culture. Our role is not to change truth but to let truth change us."

His brothers, twins Alex and Brett, lead the youth movement The Rebelution.Rebelution is a neologism defined by its creators as "a teenage rebellion against the low expectations of an ungodly culture." Harris resides in Gaithersburg, Maryland with his wife Shannon and their three children.