On May 13, 1939, Lawson was sent to the St. Louis Browns as part of a 10-player trade that sent Bobo Newsom to the Tigers. After the trade, Lawson appeared in 36 games, 14 as a starter, for the 1939 Browns, compiling a 3–7 record with a 5.32 ERA. In 1940, Lawson's second season with the Browns, Lawson appeared in 30 games, two as a starter, and compiled a 5–3 record with a 5.13 ERA. He also ranked eighth in the American League with four saves.