Birnbaum interned for Goldman Sachs in the summer of 1992 as a collateral mortgage obligations analyst.[2] After graduating Wharton he remained with the firm, eventually leading Goldman's structured products group.[4]
In 2007, Birnbaum had reportedly generated $3.7 billion for the bank by short-selling the subprime mortgage market, for which he was given a bonus of $10 million.[5] In 2010, Birnbaum testified before the U.S. Senate Permanent Subcommittee on Investigations regarding the firm’s role in the subprime mortgage crisis.[3]
After 14 years of working for Goldman, Birnbaum left in 2008 to form Tilden Park Capital Management, named after Tilden Regional Park near where he had grown up in Oakland, California. Co-founded with former Goldman colleague Jeremy Primer and fellow University of Pennsylvania alum Sam Alcoff, the firm started investing in February 2010 with a $25 million (~$35 million in 2024) managed account.[3][6]