 Born in San Juan, Puerto Rico, Pieras obtained a Bachelor of Arts degree from Catholic University of America in 1945 and his Juris Doctor from Georgetown University Law Center in 1948.He served as a Second Lieutenant in the United States Army immediately after World War II, from 1946 to 1947.He entered private practice in Hato Rey, Puerto Rico in 1949, spending 1954 in San Juan, before returning to Hato Rey.