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.