Castro attended Thomas Jefferson High School in San Antonio, where he played football, basketball and tennis; he also collected trading cards. He skipped his sophomore year and graduated in 1992, ranking ninth in his class. He had received an offer to play tennis at Trinity University, an NCAA Division III school in his hometown, but chose to attend Stanford University.

Castro graduated from Stanford in 1996 with a bachelor’s degree in political science and communications. He said he began thinking about entering politics while at Stanford, where he and his brother launched their first campaigns and won student senate seats, tying for the highest number of votes. Castro has credited affirmative action for his admission into Stanford, telling The New York Times, “Joaquin and I got into Stanford because of affirmative action. I scored 1210 on my SATs, which was lower than the median matriculating student. But I did fine in college and in law school. So did Joaquin. I’m a strong supporter of affirmative action because I’ve seen it work in my own life”. Between his sophomore and junior years, Castro worked as an intern at the White House during the presidency of Bill Clinton.

Castro entered Harvard Law School in 1997 and graduated with a Juris Doctor in 2000. His brother graduated from both schools with him. After law school, the two brothers worked for the law firm Akin Gump Strauss Hauer & Feld before starting their own firm in 2005.

In 2001, Castro was elected to the San Antonio City Council, winning 61 percent of the vote against five challengers. At age 26 he was the youngest city councilman in San Antonio history, surpassing Henry Cisneros, who won his council seat in 1975 at age 27. Castro represented District 7, a precinct on the city’s west side with 115,000 residents. The population was 70 percent Hispanic and included a large number of senior citizens. As a councilman from 2001 to 2005, he opposed a PGA-approved golf course and large-scale real estate development on the city’s outer rim.

Castro ran for mayor of San Antonio in 2005 and was widely viewed as the front runner in a field that also included retired judge Phil Hardberger and conservative city councilman Carroll Schubert. He was defeated by approximately 4000 votes when Hardberger received 51.5% of the votes in the runoff. Following his election defeat, Castro established his own law practice.

In 2007, Castro married Erica Lira, an elementary school teacher. They had a daughter in 2009 and a son in December 2014. He is Catholic. He is not a native Spanish speaker, but he began learning the language in 2010 while serving as mayor of San Antonio. He also studied Latin and Japanese in school.

Castro ran for mayor of San Antonio again in 2009. Castro hired Christian Archer, who had run Hardberger’s campaign in 2005, to run his own 2009 campaign. Castro won the election on May 9, 2009 with 56.23% of the vote, his closest opponent being Trish DeBerry-Mejia. He became the fifth Latino mayor in the history of San Antonio. He was the youngest mayor of a top-50 American city. Castro easily won re-election in 2011 and 2013, receiving 82.9% of the vote in 2011 and 67% of the vote in 2013.

Castro has been an advocate for LGBT rights and, as mayor, opposed the law in Texas (later overturned by the U.S. Supreme Court) that denied legal recognition to same-sex marriages. He is also a member of Washington D.C. based think tank the Inter-American Dialogue. Castro was the first San Antonio mayor to serve as the grand marshal of the city’s Pride Parade in 2009 and in 2011 led a push to offer domestic partner benefits in the city. In 2012, he joined mayors across the country in signing the “Mayors for the Freedom to Marry” petition for same-sex marriage equality.

In 2010, Castro created SA2020, a community-wide visioning effort. It generated a list of goals created by the people of San Antonio based on their collective vision for San Antonio in the year 2020. SA2020 then became a nonprofit organization tasked with turning that vision into a reality. Castro also established Cafe College in 2010, offering college guidance to San Antonio-area students. In 2012 he led a voter referendum to expand pre-kindergarten education. Castro persuaded two of the most prominent businessmen in San Antonio, Charles Butt and Joe Robles, to lead an effort to pass a $30 million sales tax to fund the pre-kindergarten education program.

In March 2010, Castro was named to the World Economic Forum’s list of Young Global Leaders. Later that year, Time magazine placed him on its “40 under 40” list of rising stars in American politics.

Castro gained national attention in 2012 when he was the first Hispanic to deliver the keynote address at a Democratic National Convention in Charlotte, North Carolina. Following the 2012 elections, Castro declined the position of United States Secretary of Transportation, partly with an eye on running for governor of Texas after 2017. However, in 2014, Castro accepted President Barack Obama’s offer of the position of United States Secretary of Housing and Urban Development. Castro resigned as mayor effective July 22, 2014, so that he could take up his duties in Washington. The San Antonio City Council elected councilmember Ivy Taylor to replace him.

On May 22, 2014 the White House announced Castro as the nominee to be the next secretary of Housing and Urban Development (HUD) by President Barack Obama. He was confirmed by the Senate on July 9, 2014 by a vote of 71-26 and replaced Shaun Donovan, who was nominated to be the Director of the Office of Management and Budget. He took office on July 28, 2014. Following the announcement, Castro was discussed as a potential nominee for vice president for the Democratic Party in the 2016 presidential election.

On July 28, 2014, his first day in office, Castro was honored at a reception called “Celebrating Latino Cabinet Members” hosted by the Congressional Hispanic Caucus Institute.

On October 15, 2015, Castro endorsed Hillary Clinton for president. When Clinton was asked if Castro could be her pick for vice president, she said, “I am going to look really hard at him for anything because that’s how good he is.” Discussion of Castro as a candidate to run on the Democratic ticket with Hillary Clinton increased markedly in January 2016, as the Iowa and New Hampshire primaries approached. In late January, Castro began to campaign for Clinton in Iowa, a move interpreted as a test of his appeal to the electorate. In July 2016, the U.S. Office of Special Counsel issued a finding that Castro had violated the Hatch Act by commenting on the 2016 campaign while giving an interview in an official capacity; Castro admitted the error and ordered his team to improve training on the Hatch Act.

Upon exiting office in 2017, Castro’s final memo outlined various accomplishments of the department under his leadership. These areas included HUD’s work to stabilize the housing market, rebuild communities struck by natural disasters through a $1 billion National Disaster Resilience Competition, expansion of lead safety protections in federally assisted housing, and the Affirmatively Furthering Fair Housing rule to “finally fulfill the full obligation of the Fair Housing Act.

Castro has stated that he is “not going to take any PAC money” as a presidential candidate, and has encouraged others to do the same. He had however formed a PAC (Opportunity First) in 2017 which mostly covered his running expenses while also donating to several dozen “young, progressive” Democratic politicians.

In 2018, Castro was named as the Dean’s Distinguished Fellow and Fellow of the Dávila Chair in International Trade Policy at the Lyndon B. Johnson School of Public Affairs.

In October 2018, Castro published his memoir, An Unlikely Journey: Waking Up from My American Dream through Little, Brown and Company.

In 2018, Castro visited the first in the nation New Hampshire primary state, and delivered the commencement address at New England College in Henniker, New Hampshire, on May 12, 2018. Castro stated that he would make his decision on whether to run in 2020 after the November 2018 mid-term elections. On December 12, 2018, Castro announced the formation of an exploratory committee. The next day, during an episode of The Late Show with Stephen Colbert, Julián’s brother Joaquin (during a joint appearance by both brothers) stated that he confidently believes that Julián will be running for president.

Castro formally announced his candidacy for the 2020 presidential election on January 12, 2019 at a rally in San Antonio. His brother, Congressman Joaquín Castro, and their mother introduced him at the rally. Castro would have been the first Democratic presidential nominee since 1924 to not have first served as vice president, governor or senator, and the first Hispanic or Latino nominee for president. He was the first Texan in the 2020 race and would have been the third-youngest president if elected. In his announcement, Castro emphasized Medicare-For-All, universal pre-K, and a pathway to citizenship for undocumented immigrants as part of comprehensive immigration reform. In 2019, he purchased a Fox News ad in order to speak directly to Donald Trump about the El Paso shooting. Despite, emphasizing Medicare-For-All, his health care plan was a public option.

Castro supports a path to citizenship for most undocumented residents of the US, has opposed President Trump’s “border wall” plan, and has said that the U.S. Immigration and Customs Enforcement agency needs to be “reconstituted” and that illegal immigration should be treated as a civil offense instead of a criminal one. Additionally, he asserted in the first Democratic primary candidate debates on June 26, 2019 that he would repeal Section 1325 of Title 8 of the U.S. criminal code, which would decriminalize illegal entry into the U.S., rendering unlawful entry a civil offense instead of a criminal one.

Castro suspended his presidential campaign on January 2, 2020. “¡Ganaremos un día!” he said in Spanish, which translates to “One day we’ll win!”

On January 6, 2020, Castro endorsed Senator Elizabeth Warren for the Democratic Party nomination for president. The following night, January 7, 2020, Castro gave a speech formally supporting Warren during her campaign rally in Brooklyn, New York. Castro was a ‘partner’ in Elizabeth Warren’s presidential campaign but failed to win Texas for Warren. Castro’s political future is unknown. On June 2, 2020, Castro endorsed presumptive Democratic nominee Joe Biden.

In October 2020, Castro joined the Board of Directors of the Center for American Progress, a liberal think tank founded by John Podesta.