JuliánCastro在德克萨斯州的圣安东尼奥长大，他的同胞孪生兄弟JoaquínCastro，比他年轻一分钟。他的父母从未结婚，但在卡斯特罗和他的兄弟出生几年后仍然在一起。这对夫妇参加了奇卡诺运动;卡斯特罗的父亲杰西古兹曼是一名活动家和数学老师，他的母亲罗西卡斯特罗是参与政党La Raza Unida的政治活动家。她担任该组织的Bexar县主席，帮助登记人民投票和组织政治活动。她最终在1971年对圣安东尼奥市议会投标失败。在一次采访中，罗西卡斯特罗告诉德克萨斯州观察家，当朱莉安和华金长大后，她花了大部分时间来赚取足够的钱来筹集他们作为单身母亲。但她仍然保持政治活跃。意识到他们母亲的牺牲，Julián和JoaquínCastro都在学校表现出色。 JuliánCastro在Thomas Jefferson高中踢足球，网球和篮球，并于1992年毕业。他和他的兄弟分别获得斯坦福大学和哈佛法学院的入学资格，分别于1996年和2000年毕业。 JuliánCastro在帮助他进入斯坦福大学时肯定采取肯定行动，指出他的SAT成绩没有竞争力。
Julián Castro grew up in San Antonio, Texas, and his compatriot twin brother Joaquín Castro was a younger than him. His parents never married, but they remained together after Castro and his brother were born a few years later. The couple participated in the Chicano campaign; Castro’s father, Jesse Guzman, was an activist and math teacher, and his mother, Rosie Castro, was a political activist in the party La Raza Unida. She serves as the chairman of the organization’s Bexar County, helping to register people to vote and organize political activities. She eventually failed to bid for the San Antonio City Council in 1971. In an interview, Rosie Castro told Texas observers that when Julian and Joaquin grew up, she spent most of her time earning enough money to raise them as single mothers. But she remains politically active. Aware of the sacrifice of their mother, both Julián and Joaquín Castro performed well at school. Julián Castro played football, tennis and basketball at Thomas Jefferson High School and graduated in 1992. He and his brother were admitted to Stanford University and Harvard Law School respectively, and graduated in 1996 and 2000 respectively. Julián Castro definitely took affirmative action when he helped him enter Stanford University, pointing out that his SAT scores were not competitive.