Birnbaum played in the U.S. Soccer Development Academy for Southern California club Pateadores. He played five years of college soccer at the University of California, Berkeley between 2009 and 2013, though redshirted the 2012 season. While at college, Birnbaum also appeared for USL PDL club Orange County Blue Star in 2010 and 2011.
In 2012, he won a silver medal with the U.S. team in the 2012 Pan American Maccabi Games in Brazil.
Birnbaum was selected in the first round (2nd overall) of the 2014 MLS SuperDraft on January 14, 2014 by D.C. United. He was loaned out to their USL Pro affiliate Richmond Kickers in March 2014, making his professional debut on April 5, 2014 in a 3–1 win over the Pittsburgh Riverhounds.
Following an injury to Jeff Parke, Birnbaum became United's starting center back alongside Bobby Boswell. Birnbaum was named a finalist for the 2014 MLS Rookie of the Year, but lost to Tesho Akindele. Birnbaum scored his first goal for United on February 26, 2015, in a CONCACAF Champions League quarterfinal leg against Alajuelense.
Birnbaum played for the United States U18 and United States U20 national teams in 2008 and debuted for the United States men's national soccer team on January 28, 2015. His first goal for the USMNT was a 90th-minute winner versus Iceland on January 31, 2016, a game in which he had already provided an equalizing assist to Michael Orozco to make it 2-2.
Steve is available for team mentorship only! If you have any interest in doing in person or virtual team mentorship, please reach out!