Mr. Sweeney was elected to the New York State Assembly in a special election held March 1988.

Actively involved in the area of health care, Assemblyman Sweeney has authored legislation dealing with health insurance and care for diabetes and breast cancer. In 1995, Sweeney received the National Public Policy Leadership Award from the American Diabetes Association. Assemblyman Sweeney was the sponsor of landmark legislation that requires that most health insurance contracts and health maintenance organizations in New York provide coverage for diabetes self-management education, equipment and supplies.

Assemblyman Sweeney sponsored the legislation authorizing the creation of the Broad Hollow Bioscience Park, a collaborative effort between Farmingdale State University and Cold Spring Harbor Laboratory that aids in the development and advancement of academic and hi-tech industries and which offers many students the opportunity of internships.

As Chairman of the Legislative Commission on Science and Technology, Assemblyman Sweeney sponsored and promoted legislation on genetic testing that established New York State as a leader in developing public policy on the issue.

A leader in economic development efforts, Assemblyman Sweeney has successfully worked to establish programs that assist small business, especially defense-dependent manufacturers and high-tech businesses. He received the 1995 Initiative Award from ADDAPT, a consortium of over 100 downstate manufacturing firms, and a 2001 award from the Environmental Business Association.

As Chairman of the Sub-committee on Volunteer Ambulance/Emergency Services, Sweeney authored new laws improving the service award program for volunteer ambulance workers, establishing a pilot program to extend the time for re-certification, and authorizing an emergency service worker's memorial in Albany.

As Chairman of the Assembly Local Government Committee, Assemblyman Sweeney was hailed for creating a Statewide 911 wireless program. He oversaw the adoption of municipal cooperation and consolidation statutes, and passed legislation to encourage the recruitment and retention of volunteer ambulance workers and firefighters. He initiated the annual Volunteer Fire and Ambulance Day, which focuses on the needs of these brave volunteers.

In 2007, Assemblyman Sweeney became Chairman of the Assembly Committee on Environmental Conservation. The Committee on Environmental Conservation has jurisdiction over legislation affecting State environmental policy. The primary concerns of the Committee are pollution prevention and control, resource management, and environmental quality issues.

Sweeney has been a member of the Environmental Conservation Committee since being first elected and has shepherded numerous bills into law concerning environmental issues. In his first year as Chair of the Committee Sweeney pushed through legislation to increase the Environmental Protection Fund. The additional funding for the EPF will fuel substantial increases in EPF programs such as municipal parks, waterfront revitalization, municipal recycling, open space land acquisition, farmland protection, as well as wastewater treatment projects, invasive species management and ocean and Great Lakes initiatives. Assemblyman Sweeney also successfully shepherded legislation into law placing vehicle global warming labels on new cars, created sea level rise and invasive species task forces, and worked to increase staff at the Department of Environmental Conservation and create the Climate Change Office in DEC. As a result Sweeney has been named “Legislator of the Year” by EPL/Environmental Advocates, a nonprofit organization that has worked on behalf of environmental issues in New York State for 40 years. Additionally, the New York League of Conservation Voters named Sweeney an Environmental Star of 2007 “for his efforts to ensure that all New Yorkers can enjoy clean air, more open space and a better quality of life.” The Healthy Schools Network presented Sweeney with its Healthy Schools Hero Award for 2007 for his work making schools environmentally safer.

In 2009, Assemblyman Sweeney received Audubon New York's prestigious William Hoyt Environmental Excellence Award, which is presented annually to an elected official in recognition for work above and beyond the normal call of duty to advance critical solutions to the most pressing environmental issues.

Assemblyman Sweeney annually sponsors, with the Town of Babylon, a Veteran's Awareness Week. This week, in honor of Americans who honorably served their country, helps us to remember our veterans. At the request of a high school classmate blinded while serving in Vietnam, Assemblyman Sweeney successfully worked to double New York's Blinded Veteran's Annuity, which hadn't been changed in 80 years.

A strong supporter of higher education, Assemblyman Sweeney received the 1996 Friend of SUNY Award and the 2001 Friend of SUNY Farmingdale Award.

In 2000, Assemblyman Sweeney was named one of the "100 Most Influential People" of the 20th century in the Town of Babylon and in 2005 earned recognition as "one of Albany's most prolific legislators" by the Gannett News Service for his ability to sponsor bills that get signed into law.