Peter Henner established his own law practice on August 1, 1984, after previously working as Assistant Counsel for the New York State Assembly Majority and as the General Counsel for Council 82 AFSCME, representing 20,000 law enforcement employees in New York State.  The practice was maintained as a full-time solo practice for 28 years, until Peter decided to limit his practice in order to devote time to other activities.

As a solo practitioner, Peter practiced labor law, representing employees and unions.  He not only represented individual employees in grievance disputes, but also handled miscellaneous matters for both private and public sector unions, represented employees in employment discrimination and First Amendment retaliation lawsuits, and represented whistleblowers in a variety of administrative and judicial proceedings.

Peter also determined to become an environmental lawyer.  He became one of the most prolific litigators of citizen suits under the Clean Water Act, both for Atlantic States Legal Foundation and the Alliance for Environmental Renewal, an organization that he helped establish in 1995.  These cases enabled environmental organizations to vindicate important environmental interests, while funding their efforts through attorneys’ fees recovered from polluters.  In addition to commencing citizen suits, Peter handled a number of high-profile cases involving claims that governmental bodies failed to comply with the provisions of the New York State Environmental Quality Review Act, including challenges to landfill expansions and the sale of the Indian Point Nuclear Facility.  Peter generally represented environmental organizations or municipalities seeking to prevent obnoxious and ill-considered projects.

Peter has been noted for his ability to quickly grasp complex situations, including cases that involve an extensive body of technical material, and to apply the relevant law.  For example, he brought challenges to the deregulation of the electric industry in New York State, which required a comprehensive understanding of utility regulation, and the nonrenewal of a charter school, which required extensive knowledge of educational statistics used to review school performance, as well as numerous environmental matters that required a clear understanding of environmental and scientific issues.

Peter's clients have included his home community of Clarksville in a successful effort to stop the United States Postal Service from closing the local post office, municipalities that are seeking to become  "green" cities, community groups seeking to stop ill-considered development, the civil servant fired for criticizing the County Executive, honest police officers whose lives were threatened by a corrupt PBA,  a female police Lieutenant seeking to smash the blue glass ceiling of a large urban police department, unions trying to organize union-busting companies, whistleblowers seeking to expose government fraud, and victims of environmental outrage anywhere in the world, as well as many people and groups who have been mistreated by arbitrary governmental actions, or oppressed by corporate greed and arrogance.