Swanstrom attended studied law at New York University, graduating from there in 1878. He was also a student in the law firm Miller, Peet & Obdyke. The firm later became Bristow, Peet & Opdyke, headed by Secretary of the Treasury Benjamin Bristow. After graduating, he quickly began a general practice. He shared a law office with John E. Miller at 20 Nassau Street and became a prominent Brooklyn lawyer. After becoming Borough President, he took on a partner and formed the law firm Swanstrom & Keyes.
