A member of the Republican Party, DeMaio represents the 75th State Assembly District, encompassing North and East San Diego County.

[5] DeMaio previously served a single term as a member of the San Diego City Council, representing District 5 from 2008 to 2012.

[9] His mother died of breast cancer in 1990 when he was 15 years old; his abusive father abandoned the family two weeks prior to her death.

[11] He graduated in 1993, then attended Georgetown University's School of Foreign Service, where he received a degree in international politics and business in 1996.

[18] In 2000, DeMaio joined the Reason Foundation to lead a project to develop a bipartisan management improvement plan for the incoming Presidential Administration.

C-Span broadcast a series of Townhall hosted by DeMaio that eventually led to a management reform plan that was accepted by incoming President George W.

[24] In April 2015, DeMaio joined KOGO Radio (AM-600), first as a co-host with Bob Sullivan of a midday radio show (1-4 pm) with Bob Sullivan, and then starting in November 2015 as a solo host of his own show, "The DeMaio Report," during the afternoon drive (3-6pm).

[25] In August 2019, he took a leave of absence to launch his 2020 Congressional race and began airing his own Podcast as a way to stay connected with his followers.

In December 2023, DeMaio again took a leave from KOGO to launch his 2024 Assembly race, but continued his Podcast as a way to stay connected with his followers.

[5] DeMaio ran for the termed-out Brian Maienschein's District 5 San Diego City Council seat in the nonpartisan 2008 election.

However, the measure was rejected by the county registrar of voters after a random sample concluded that DeMaio had not gathered enough valid signatures.

[45] In the June 5 primary he placed first with 31.42% of the vote and advanced to a runoff election against U.S. Representative Bob Filner in November.

[52][53] In September 2013, he considered running for mayor of San Diego in a November 2013 special election, called because of Filner's resignation, but decided to stay in the race for Congress.

[54] The month before the primary election, the campaign office of DeMaio was broken into;[55] an affidavit signed by a San Diego Police Department detective, unsealed after the November 2014 election, stated the belief that Todd Bosnich was the culprit of the campaign office burglary.

[57] In the June 2014 primary, he came in second to Peters with 36% of the vote, ensuring DeMaio a place on the ballot in the November 2014 general election.

[61] DeMaio denied the allegations, saying they were made after Bosnich had been terminated for plagiarism and subsequently vandalized campaign headquarters.

"[63] On October 20, the San Diego County District Attorney declined to file charges against either DeMaio or Bosnich.

[61] In June 2015, Bosnich admitted that he had faked threatening emails he claimed were from DeMaio, and pleaded guilty to one count of obstruction of justice – a federal felony count;[65] In November 2015, US District Judge Larry Alan Burns sentenced Bosnich to probation, community service, a fine, and mental health counseling.

[67][68] The US Attorney's Office stated Bosnich's sent the fabricated emails to himself "to bolster his claims that DeMaio was threatening him to remain silent about the alleged sexual harassment.

Voice of San Diego attempted to follow up on the accusation in December 2014, but did not find anything conclusive supporting either Harper or DeMaio.

[76] On August 5, 2019, DeMaio announced his candidacy for California's 50th congressional district in the 2020 election, then occupied by Congressman Duncan Hunter, a fellow Republican who was indicted for misusing campaign funds a month prior.

[80] The March 3 primary election for the vacant seat pitted DeMaio against Democrat Ammar Campa-Najjar and Republicans Darrell Issa and Brian Jones.

[83] DeMaio ran for the District 75 seat against another Republican, Lakeside School Board Trustee Andrew Hayes.

"[92] He proposes national legislation that would encourage Americans to carry weapons, after undergoing background checks, to combat mass shootings.

Details include mandatory self-defense training in all schools, a national concealed-carry permit program, and better background checks.