Born and raised in Texas, Don Haslam earned a degree in economics at the University of Chicago, where he was quarterback and pitcher on NCAA teams. After graduation, he traded financial futures on the floors of the Chicago Board of Trade and the Chicago Mercantile Exchange. He joined several large banks as a bond, currency and derivatives trader, including NationsBank Texas, where he also managed a bond portfolio in excess of $50 million. He put himself through the University of Tulsa Law School while Vice President and foreign exchange trader at Liberty Bank & Trust in Tulsa.

Haslam joined Riggs, Abney, Neal, Turpen, Orbison and Lewis, one of Oklahoma's largest law firms, where his work focused on business transactions for large clients such as EMSA, the Tulsa County Criminal Justice Authority, Case Properties, and others.

From 2007 to 2011, Haslam was public defender in three SE Oklahoma counties, defending in excess of 500 felonies and 400 misdemeanors. He has served as Chancellor for the Diocese of the Southwestern States [the Anglican Church] and board member of Domestic Abuse Helpline for Men and Women, a national organization that assists people in abusive relationships and rejects the myth that these relationships most often afflict women.