He joined Bethlehem Steel in 1959 as a member of the corporation's Loop Course management training program. Subsequently Mr. Millenbruch advanced through various levels of responsibility. He was elected assistant controller in 1977, controller in 1980 and to the additional position of vice president, accounting, in 1982.

In 1985, Mr. Millenbruch was elected senior vice president and group executive, Steel-Related Group, and assumed responsibility for Manufactured Products, Marine Construction, Natural Resources and Information Services. In March 1986, Mr. Millenbruch was elected senior vice president and chief financial officer, and continued to serve as group executive of the Steel-Related Group. He was elected to the Board of Directors in June 1991. He was elected executive vice president and chief financial officer, effective November 1, 1992, and to the additional position of treasurer, effective September 1, 1994.

Additionally, effective August of 1999, Mr. Millenbruch was named vice chairman. In this capacity, he participates in the development of strategic initiatives, as well as manages the overall responsibility for Bethlehem’s financial activities.

Mr. Millenbruch is a member of the American Iron and Steel Institute and is past chairman of its Committee on Accounting. He is also a member of the Financial Executives Institute and a past member of its Committee on Corporate Reporting and the Board of Trustees of its Financial Executive Research Foundation. He serves on the Chief Financial Officers’ Advisory Council of the Metropolitan area of New York City and is a member of the Delaware Valley Financial Executives Group. He is a member of Saucon Valley Country Club, and a former member of its Board of Governors and Chairman of its Audit Committee. He is a member of the Advisory Board of the Minsi Trails Council of the Boy Scouts of America and a former Treasurer and member of the Executive Committee of the Board. He currently serves as a member of the Board of the National Museum of Industrial History, as Vice President of the Board of Trustees of the Allentown Art Museum, as a member of the Executive Committee of the Board of Associates for Muhlenberg College and is a member of the Finance Advisory Board of Kansas State University where he has previously served as President of the Board.