Block formerly served as the executive director of the Wisconsin chapter of Americans for Prosperity, a free-market advocacy organization. Block serves on the Board of Directors of the MacIver Institute and the First Freedoms Foundation. In 2008, Block was appointed a member of the Wisconsin Advisory Committee to the U.S. Commission on Civil Rights.
Block began his political career at age 18. In April 1974, he became the first 18-year-old Wisconsin history to be elected to political office when he was elected to the Winnebago County Board of Supervisors.
Block served as the State Director of Americans for Prosperity in Wisconsin from 2005-2010, when he left to work for Herman Cain's presidential campaign.
Block started his political career as a Legislative Assistant to the late Congressman Bill Stieger, a position that ended with Steiger's untimely death in 1979. After Stieger's death, Block worked with NCR as a marketing manager. He started the consulting firm "Mark Block and Associates".
Americans for Prosperity
A notable focus of Block's time at the Americans for Prosperity chapter in Wisconsin was organizing taxpayer rallies in Madison, Wisconsin and elsewhere. On October 17, 2007, over 800 people from all over Wisconsin flocked to Madison to demand a no new tax increase state budget when the state's budget was 100 days overdue.
On April 15, 2009, Block organized the largest taxpayer rally in Wisconsin history, with over 8,800 people attending the Madison Tea Party rally. The 2009 rally was the largest protest of any kind held on the steps of Wisconsin's State Capitol in over 44 years.
Mark Block managed the 1997 election campaign of Wisconsin Supreme Court Justice Jon P. Wilcox. Wilcox won the election, and Block was later charged with illegal coordination with the political activities of Wisconsin Citizens for Voter Participation. During that period the constitutionality of the Milwaukee Parental Choice Program was facing a legal challenge. If Wilcox's opponent Walt Kelley had been elected the Milwaukee program might have been ruled unconstitutional. 
Block reached a settlement to end litigation by paying a $15,000 fine and agreeing to a three-year ban on volunteer or paid political activity. Brent Pickens, a former Wisconsin State Assembly staffer who was running Wisconsin Citizens for Voter Participation, ended related litigation against him by agreeing to pay a $15,000 fine. Pickens also agreed to avoid political activity for five years.
Government Accountability Board investigation
Block, eleven years removed from the Jon Wilcox controversy, faced another Wisconsin Government Accountability Board investigation over whether a mailer sent to Americans for Prosperity members during the 2008 Wisconsin Supreme Court election.
In October 2008, the Wisconsin Government Accountability Board cleared Block of wrongdoing in a unanimous 6-0 vote, affirming the claims of Americans for Prosperity and his legal team that the original intent of the letter was legal communication.
West Bend School referendum
In 2007, The West Bend Area School District attempted to pass a $120 million school referendum. This would have been the largest school referenda in Wisconsin history. Americans for Prosperity organized a phone call campaign to communicate with voters in the district about the referenda's tax ramifications. The school district's attorney responded to a request from It's Time, a West Bend group supporting passage of the referendum, to investigate whether the message violated state law. The calls were placed Friday before the referendum in November 2007.
Washington County District Attorney Todd Martens ruled in favor of Americans for Prosperity, writing, "I believe the language in the call comes dangerously close to express advocacy...Although it appears that some of the information in the call is misleading and a distortion of the true financial impact of the referendum on taxpayers, the veracity of the political speech is not the issue in determining whether Americans for Prosperity is required to register as a political committee."
State election law requires anyone expressly promoting the passage or defeat of a referendum question to register to campaign, according to the state Elections Board.
Block has said the message was intended to educate West Bend residents.
In the West Bend referendum, Americans for Prosperity's telephone message said the building plan would cost "$574,000 for each child" added to the district since a 2001 referendum.
West Bend School Superintendent Patricia Herdrich described the message as "inaccurate and misleading.".