For 39 years, prior to his retirement, Professor Williamson was the Director of Graduate Tax Program (MST). He taught graduate classes that included Partnership Taxation, Corporate Taxation, International Taxation, Estate and Gift Taxation, Income Taxation of Trusts and Estates, Advanced Tax Accounting, Tax Procedure and Research and Special Topics in Taxation.

Adjunct Professor of Law. Courses taught to third year law students include Partnership Taxation and Law and Accounting.

Senior Manager for International Taxation at the National Tax Practice Office. Provided technical advice on international tax issues to the Washington Firm's operating offices.