Mr. Charnas advises businesses on a broad range of individual, corporate and partnership matters, including business formations, operations, acquisitions, reorganizations, taxes, and controversies with the IRS (where he has served as a senior attorney and special assistant to the chief counsel) and the Justice Department. His clients are in numerous industries, including health care, retail, motor fuels and professional services. His representation of businesses includes advising business owners on succession and complex estate planning issues. A significant portion of his practice involves representing nonprofit organizations.

In addition to training associate lawyers in the firm, he is an adjunct professor at Georgetown Law Center, where he teaches a course on S corporations and limited liability companies in the LLM program in tax. He is a frequent contributor to publications of the national trade associations he represents. He co-authored and edited a comprehensive handbook for tax-exempt organizations regarding public disclosure requirements, as well as a handbook for independent insurance agents regarding business structures and taxation.

He is a member of the ABA Section of Taxation and Section of Business Law; the D.C. Bar Section of Taxation and the Estates, Trusts, and Probate Law Section; the Inter-American Bar Association; and the J. Edgar Murdock Inn of Court. He serves as secretary, general counsel, and a director of the S&R Foundation; secretary and general counsel to the James J. Norris Foundation for Migrants and Refugees and the Divino Amore Foundation; as general counsel to the National Catholic Community Foundation and the Bill Raskob Foundation; and as a member of the Board of Visitors of the Ohio University Honors Tutorial College.