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D. Sean McMahon – Partner


D. Sean McMahon is the founding partner of McMahon & Tivnan, PC and a lecturer at Boston University School of Law’s Graduate Tax Program where he teaches IRS practice and procedure. Previously he served as a Senior Attorney with the IRS Office of Chief Counsel and as a Special Assistant United States Attorney for Massachusetts and New Hampshire where he was responsible for tax cases. Sean has more than twenty-five years of experience investigating and defending IRS and state tax matters and excels at resolving difficult and complex tax disputes. He counsels clients through examinations and liability disputes and, when necessary, takes disputed issues to court. Sean has been named a Massachusetts Super Lawyer among numerous other best lawyer lists.

As a former IRS attorney, Sean offers clients unique insights on the real risks presented in disputes with the IRS and planning for complex transactions. He advises clients on the tax considerations of corporate, real estate, partnership, securities, and option transactions. Sean also has extensive experience assisting individuals with foreign transaction reporting and penalty disputes, including foreign bank accounts and FBAR requirements.

Tax Attorney Experience

Sean represents individuals, estates, and businesses with federal and state tax issues from audit through the collection. His practice includes:

  • Examination defense, including calculation of correct tax liability, and minimizing penalties and interest
  • Collections, including installment agreements, offer in compromises, and innocent spouse
  • Appeals, including administrative appeals through the Office of Appeals and appellate courts
  • Voluntary Disclosure, including helping to bring taxpayers who have not filed returns in several years into compliance
  • Foreign Bank Accounts and FBAR Requirements, including representing U.S. citizens with dual citizenship with unreported foreign bank accounts and delinquent FBAR returns, and voluntary disclosure programs
  • Foreign Trust and Foreign Corporate interest informational returns and related non-filing penalty defense
  • Tax Litigation, including litigating tax disputes before the U.S. Tax Court, U.S. District Courts, and the U.S. Court of Federal Claims

Additionally, Sean teaches a graduate-level class on Tax Practice and Procedures at Boston University School of Law.

Topics covered in the class include:

  • Administration of the Internal Revenue Code by the Internal Revenue Service
  • Study of the administrative processing of tax returns
  • Handling of audits, statutes of limitations, assessment of deficiencies and penalties, claims for refund, hearings before the Appeals Office, closing agreement, tax liens, tax collection procedures, and civil and criminal aspects of tax fraud
  • Introduction to aspects of litigation in the Federal District Court, U.S. Court of Federal Claims, and U.S. Tax Court

Previous Tax Experience

Internal Revenue Service:

  • Senior Attorney IRS Office of Chief Counsel
  • Special Assistant United States Attorney for Massachusetts and New Hampshire responsible for tax cases

Professional Affiliations

  • Massachusetts Bar Association Tax Section, council member
  • American Bar Association

Speaking Engagements

Panelist, “Collection: New IRS Techniques and Key Taxpayer Strategies,” 2017 ABA 34th Annual National Institute on Criminal Tax Fraud and 7th Annual National Institute on Tax Controversy (2017)

Speaker, “Correcting United States Income Tax and Foreign Asset Reporting Problems,” Massachusetts Society of Enrolled Agents (2017)

Panelist, “Ethical Considerations in Individual and Family Representation,” ABA Section of Taxation and Section of Real Property, Trust & Estate Law, Trust & Estate Division 2016 Joint Fall CLE Meeting (2016)

Speaker, “Correcting IRS Income Tax and Foreign Asset Reporting Problems,” Kuwait (2016)

Speaker, “Correcting IRS Income Tax and Foreign Asset Reporting Problems,” Kuwait (2014)

Speaker, “Avoiding Tax Problems for a New Business,” Eighth Business Fundamentals Bootcamp (2013)

Panelist, Working with Overseas Americans and Foreign Nationals in the United States and IRS Voluntary Disclosure Program Update,” CPE Credit Program for Certified Public Accountants (2012)

Panelist, “What Every CPA Needs to Know About Civil and Criminal Tax Litigation,” CPE Credit Program for Certified Public Accountants (2011, 2010)

Panelist, “The IRS Voluntary Offshore Disclosure Initiative,” CPE Credit Program for Certified Public Accountants (2010)

Panelist, “The Madoff Mess” Massachusetts Continuing Legal Education, Inc. (MCLE), (2009)

Honors & Awards

Selected to the Massachusetts Super Lawyers list, 2013 – 2021