Thaddeus F.B. Wasielewski was born 2 December 1904 in Milwaukee, to Dr. Frank S. and Felicia H. (Baranowski) Wasielewski. He attended S.S. Cyril and Methodius Parochial School and South Division High School, and received degrees from the University of Michigan (B.A., 1927) and from Marquette University Law School (J.D., 1931). In 1931 Wasielewski entered practice as an attorney in Milwaukee. On 28 October 1939, he married Stephanie M. Gorak, daughter of Mr. and Mrs. Thomas Gorak of Milwaukee.
Following an unsuccessful campaign in 1938 for a seat in the U.S. House of Representatives, Wasielewski defeated the Republican incumbent, John Schafer, in November 1940 to represent Wisconsin's 4th Congressional District (Milwaukee). He served on the Foreign Affairs and Ways and Means Committees, and was a staunch supporter of President Franklin D. Roosevelt and the American war effort. In 1946 Wasielewski was defeated in the primary election, partially due to allegations that he had voted for antilabor legislation in the preceding Congress. When the Democratic cominee, Edmund V. Bobrowicz, was accused of having Communist sympathies and a Communist Party membership, Wasielewski entered the race as an Independent. He was third in the general election.
In 1948 Wasielewski served as a delegate to the Democratic National Convention and also was a candidate for appointment as Judge of the U.S. District Court for the Eastern District of Wisconsin. Two years later he attempted to regain the 4th District Congressional seat, losing in the primary election to Clement Zablocki. Following his earlier defeat in 1946, Wasielewski had returned to his law practice in Milwaukee, which he continued for the remainder of his life.
Wasielewski was also active in numerous fraternal, civic and religious organizations in Milwaukee. He held memberships in and served as an officer of the Polish National Alliance, Polish Roman Catholic Union, Polish Association of America, Pulaski Council, and American Committee for Relief to Poland. Wasielewski organized both the Milwaukee County Citizen's Committee (1938), an umbrella group representing all Milwaukee Polish organizations; and the Woodrow Wilson Lawyers Club, composed of Milwaukee-area attorneys of Polish ancestry.
Thaddeus Wasielewski died on 25 April 1976, and was survived by his widow, Stephanie, and five children, Francis, Stephanie Fay, Janina Clossen, Mary Parsons, and John.
