Castillo was born and raised in Chicago's Logan Square neighborhood. He attended Goethe School, a public elementary school where he discovered his love of learning and Goethe's Playground where he discovered his love of sports & formed many lifelong friendships. He realized in high school that he was different but it wasn't until attending college that he "came out" to friends and family.
An interest in the LGBTQ community and activism followed and Castillo became active in a number of organizations. In 2001, he was inducted into the Chicago LGBT Hall of Fame for his work helping advance the rights of lesbian, gay, bisexual and transgender persons. To view his work, visit: https://chicagolgbthalloffame.org/castillo-robert/ He married John Pennycuff, in 2004 when then Mayor Gavin Newsom allowed same-sex couples to marry. Pennycuff was also active in LGBTQ activism and the pair were a staple at many rallies, marches and protests from the early 90s until John's passing in 2012.
Castillo has worked to helped preserve his husband's legacy by funding and designing a Little Library at Unity Park in Logan Square. He also supported the creation of the John Pennycuff Memorial Apartments at Robert Castillo Plaza , a 88 unit housing development at 2037 N Milwaukee in Chicago which opened in 2020.
Today, Castillo remains committed to LGBTQ+ equality and hopes to spend his remaining time making sure the rights that he, John and fellow activists fought so hard for remain in place.