He resigned from the State House in 2004 to work as senior policy adviser to Republican Congressman Jeb Bradley.

He then attended Franklin Pierce Law Center in New Hampshire, where he earned a master's degree in intellectual property.

In the non-partisan municipal primary election held on September 1, 2001,[5] Guinta and George Skilioganis were the two top vote-getters in Ward 3, with 375 and 279 votes, respectively, securing them a place on the November ballot.

In the non-partisan primary held on September 5, 2005, Guinta placed second in a three-candidate field, garnering 3,760 votes to Baines' 5,168.

)[9] On November 8, 2005, Guinta defeated Baines in the general election by 528 votes (10,125 to 9,597),[10] becoming Manchester's youngest mayor in over 100 years.

He ran on a platform of improving education, increasing public safety and security, revitalizing Manchester's neighborhoods, promoting fiscal responsibility, and reducing property tax rates.

[15] Guinta was elected to a second term as mayor on November 6, 2007, defeating Democrat Thomas Donovan, a former school board member.

The paper's editorial board praised Guinta as "a tax-cutting crime fighter...[who] has pushed bureaucratic reform and improved services.

"[17] In June 2009, Mayor Guinta announced his plan to lower property taxes by reducing school funding by 7 million dollars.

[18] Guinta explained his budget by telling WMUR-TV, "We've got to find ways to be more effective, more efficient so we can keep money in taxpayers' and property owners' pockets.

Guinta's Democratic opponent, incumbent Carol Shea-Porter, had represented New Hampshire's 1st congressional district for two terms.

[30][31] In May 2015, Guinta settled the case with the Federal Election Commission involving $355,000 that had been donated to him by his parents during his first House campaign in 2010.

Senator Kelly Ayotte (R) called on Guinta to resign his House seat in light of the incident, but he refused.

[33] Guinta won the 2012 primary election handily, obtaining 84% of the vote against Republican challengers Rick Parent and Vern Clough.

[34] Shea-Porter was nominated again by the Democrats to retake the seat, and Brendan Kelly ran on the Libertarian Party ticket.

[37] Guinta has worked to place a full-service VA medical facility in New Hampshire and has emphasized veterans' homelessness within the district.

[43] Guinta has opposed the automatic cuts required by the Budget Control Act of 2011 (the "sequester") that affect defense spending, out of concern for employment at the Portsmouth Naval Shipyard.

[47] On July 22, 2012, CREDO activists, joined by Occupy movement members, staged a protest at Manchester's Northeast Delta Dental Stadium, where Guinta was holding a fund-raiser.