Early tenure
Mullin was sworn into office on January 3, 2013. In his early tenure, his record was largely partisan, according to The Oklahoman. He voted against bipartisan bills to prepare federal agencies for public health emergencies and establish a network for researching pediatric diseases, arguing that the legislation would enlarge the federal government. Despite his partisan record, Mullin advocated for negotiating as opposed to compromising. He established an early-morning workout group that included Massachusetts representative Joe Kennedy III and Hawaii representative Tulsi Gabbard. Mullin was involved in discussions involving the Affordable Care Act that preceded a government shutdown in October.
On April 8, 2014, Mullin announced that he would seek reelection in that year's election. He was challenged by the professional angler Darrel Robertson, who portrayed Mullin as beholden to special interests; Mullin defeated Robertson in the Republican primary on June 24. He defeated the Democratic nominee, the retiree Earl Everett, in the general election on November 4, but Everett was killed in a car accident hours before the election. Oklahoma Democratic Party chair Wallace Collins said the party would seek a special election but it declined to sue to secure one.
In September 2015, House Speaker John Boehner announced that he would resign, setting off discussions over the party leadership in the House. House majority whip Steve Scalise was expected to run for House majority leader. According to Politico, Mullin had begun a campaign to succeed Scalise hours after Boehner's announcement. In the days after Boehner's announcement, several representatives—including Mullin—announced that they would enter the race for majority whip. Associates of North Carolina representative Patrick McHenry, who was aligned with California representative Kevin McCarthy, believed he had already won the position. In October, McCarthy announced that he would not seek to succeed Boehner in the speakership election after the Freedom Caucus refused to support him, forcing McCarthy to remain as majority leader and Scalise as majority whip. Mullin later served as a deputy whip.
In April 2016, Mullin filed to run in that year's election. He told the Tulsa World that he would not renege on his campaign vow to serve no more than three terms. He faced a challenge from Jarrin Jackson, a United States Army veteran critical of Mullin's vote on a spending bill the previous year. As the June primary approached, the race intensified, particularly after former Oklahoma senator Tom Coburn endorsed Jackson. Mullin defeated Jackson in the Republican primary on June 28, and defeated Democratic nominee Joshua Harris-Till and independent John McCarthy in the general election on November 8.
According to the Congressional Record, Mullin traveled to Jordan for three days in August 2016. The details of his visit remained largely vague. In his confirmation hearing to serve as secretary of homeland security in 2026, Mullin said he had gone on a classified visit, like an "official travel with specific deals or specific fact-finding". Mullin said he had gone through Survival, Evasion, Resistance and Escape training with a contingency to prepare for the visit. The New York Times associated the visit with Mullin's praise of Abdullah II for sending his personal helicopter to rescue him and other Americans, as well as the Syrian civil war. By December 2016, president-elect Donald Trump had named Mullin the chair of his Native American Coalition. That month, the coalition met with Trump's transition team to work to improve relations with Native American tribes.
