He attended Mercer University in Macon, where he earned his bachelor and law degrees with honors. After he earned his Juris Doctor degree in 1966, he went into the United States Army, where he earned the rank of captain.

Nathan Deal introduced H.R. 698, the Citizenship Reform Act, which would eliminate birthright citizenship for illegal aliens in the U.S. The 14th Amendment begins "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States...." Deal's argument is that illegal aliens (and their children) are not subject to U.S. jurisdiction.

Deal spent twenty-three years in private law practice. He was also a Criminal prosecutor, a Hall County Juvenile court judge, and a Northeastern Judicial Circuit superior court judge. In 1980, he was elected to the Georgia Senate as a Democrat.

In November 1990, he was elected by his party to be the President Pro Tempore, the second highest ranking position in the chamber. Democrat Jane R. Hemmer replaced him, but was defeated by Republican Casey Cagle.

Deal was first elected to Congress in November 1992 as a Democrat, succeeding eight-term incumbent Ed Jenkins in Georgia's 9th congressional district. He was re-elected as a Democrat in 1994.

However, on April 11, 1995, shortly after Republicans assumed control of the US House of Representatives for the first time in 40 years, Deal joined the Republican Party, which was led by Speaker Newt Gingrich, a fellow Georgian. Years later, Gingrich said that Deal became a Republican because he liked what he saw in the Contract With America.

Deal's voting record was relatively moderate in his first term, getting ratings in the 60s from the American Conservative Union (ACU). He moved sharply to the right after his party switch and voted for all four articles of impeachment against Bill Clinton. From 1996 onward, he garnered ratings of 90 or higher from the ACU.

In the general election, he faced former governor and state senator Roy Barnes (D) and John Monds (L). Barnes previously won the 1998 gubernatorial election with 52% of the vote, and lost re-election in 2002 to State Senator Sonny Perdue 51%-46%. Perdue was the first Georgia Republican Governor since Reconstruction.

In November 2006, Deal was re-elected 77%–23%. His Democratic opponent was John Bradbury, a former elementary school Teacher turned truck driver. His district, already heavily Republican, became even more Republican after the mid-decade redistricting pushed it further into the Atlanta suburbs.

With the help of the Council and the Vera Institute of Justice, Gov. Deal has developed extensive performance measures to track the success of previous reform to ensure they are enhancing public safety and saving taxpayer dollars. The Atlanta Journal-Constitution reported, “Since 2007 alone, more than three-dozen such courts have opened their doors across Georgia. In the first quarter of 2014, more than 4,100 offenders were enrolled in the state’s 105 accountability courts, and many of these participants would likely be in prison without this alternative.”

Incumbent Republican Governor Sonny Perdue was term-limited in 2010. Seven candidates filed to run in the Republican primary. In the initial Republican primary in July, no candidate received the 50% threshold to win the primary outright. Georgia Secretary of State Karen Handel ranked first with 34%, qualifying for the run-off election. Deal ranked second with 23% of the vote.

In 2011, Georgia was in the midst of a Criminal justice crisis. The prison population had doubled in the past two decades to 56,000, along with the state's incarceration budget. The recidivism rate was 30 percent for adults and 65 percent for juveniles. In response, Deal commissioned the Georgia Criminal Justice Reform Council, tasked with performing an exhaustive review of the state's current system, identifying key areas of focus and providing recommendations for reforms. These areas included increased funding and support for accountability courts, overhauling the Juvenile justice system, and implementing prisoner re-entry initiatives. The Council's work resulted in bipartisan legislation that is paying dividends. Through these efforts, Georgia has avoided the need for 5,000 additional prison beds over 5 years and saved taxpayers at least $264 million. A 2014 study showed that “prison sentences imposed on African-American offenders have dropped by 20 percent.”

On July 23, 2012, the ethics commission cleared Deal of major ethics violations while finding he made “technical defects” in a series of personal financial and campaign Finance reports. In July 2012, Deal agreed to pay $3,350 in administrative fees to resolve violations of campaign Finance and disclosure laws.

On April 25, 2013, Governor Deal signed HB 349 into law, which enacted a second round of Criminal justice reforms. These reforms took a “smart on crime” approach and were based on recommendations from the Special Council on Criminal Justice Reform. This law gave those who, while locked up, have earned money toward college in the form of a HOPE Scholarship G-E-D Voucher the ability to use that money up to two years after their release. In addition, Gov. Deal reinvested $5M to create a voluntary grant program that gives communities incentives to offer judges more non-confinement sentencing options. These could include substance abuse treatment or family counseling. These methods have been proven to better reduce recidivism for low-risk offenders.

In 2014, Deal signed House Bill (H.B.) 60, the Safe Carry Protection Act, referred to by critics as the "Guns Everywhere" Law. Deal stated that gun rights through the United States Constitution's Second Amendment are important to people in Georgia. The Safe Carry Protection Act is a controversial new law to take effect on July 1, 2014, that permits licensed gun owners to carry guns into many public and private places, including churches, school property, bars, nightclubs, libraries, and some government buildings in Georgia. The law was supported by the Georgia Baptist Convention which includes 3,600 Baptist churches in Georgia in favor of increased church autonomy, but is not supported by Catholic or Episcopalian church Leaders due to their belief that it is against Jesus' teachings. By 2016, The Atlanta Journal-Constitution found that while 57% of Georgians believed that owning a gun protects people, 59% disapproved of the law itself.

Deal took office as governor on January 10, the second Monday of 2011. His second inauguration took place on January 12, 2015.

On May 3, 2016, Deal vetoed a campus carry bill that had been passed by the state legislature, after a number of state legislators refused to include exceptions for child-care centers and other places on college campuses. Had Deal signed the bill into law, it would have made concealed carrying of guns legal at every public college in Georgia, so long as the carrier was 21 or older and had a proper permit. One year later, on May 4, 2017, Deal signed a revised and stricter version of the campus carry bill into law.