Joseph M. Pratt was born in Paterson, New Jersey. He moved with his parents to Philadelphia, Pennsylvania, in 1892. He graduated from Temple University in Philadelphia in 1919. He engaged in the manufacture of industrial and marine products. He was a member of the Republican City Committee of Philadelphia from 1937 to 1946.
Pratt was elected as a Republican to the Seventy-eighth Congress to fill the vacancy caused by the resignation of James P. McGranery. He was an unsuccessful candidate for reelection in 1944. He resumed his former business pursuits in Philadelphia. He received the Republican nomination for State senator in the second senatorial district of Pennsylvania in 1946 but died in Washington, D.C. that same year while on a business trip. Interment in Arlington Cemetery in Upper Darby, Pennsylvania.