Steven Barker joined Cornerstone as a Principal in our Oklahoma office in November 2025.

Prior to Cornerstone he served as Assistant Municipal Counselor for the City of Oklahoma City. In this role, he represented the city in eminent domain and land use matters, advised the City Manager's Office and Planning Department and supported key strategic initiatives, including securing a 25-year lease agreement with the Oklahoma City Thunder.

Previously, Steven served as General Counsel at Coop Ale Works, providing nationwide regulatory guidance to support product distribution. In this role, he managed all federal and state compliance, negotiated agreements with distributors and oversaw the company's licensing portfolio.

Earlier in his career, Steven worked with the Oklahoma Alcoholic Beverage Laws Enforcement Commission, where he held leadership roles as Deputy Director and General Counsel. He has also served as an Assistant Attorney General in the Oklahoma Attorney General's Office and as Assistant General Counsel with the Oklahoma Tax Commission. Steven currently serves as Chief Municipal Judge for the City of Warr Acres, where he presides over arraignment, juvenile and trial dockets.

Steven holds a Juris Doctor from Oklahoma City University School of Law and a Bachelor of Arts in Political Science from Pittsburg State University.

Mark Yates joined Cornerstone in July 2021 as a principal in the firm's Oklahoma City office. His experience in public policy includes working with various industries through statewide campaigns, legislative engagement, media relations, and public speaking. Mark has been successful navigating complex and challenging initiatives on behalf of the clean energy and agriculture industries.

Mark previously served as vice president at Advanced Power Alliance (APA) where he represented leading domestic and international companies in the renewable energy sector. While in this role, he managed the association's regional public policy strategy, communications strategy, and membership recruitment efforts. Mark served a major role in shaping legislative and regulatory policy within the most important energy region in the country.

Prior to APA, Mark worked at the Oklahoma Farm Bureau, where he led membership recruitment initiatives, mobilized a statewide grassroots campaign, and lobbied for rural issues on behalf of the nearly 90,000 farmers and ranchers.

Before his time with the Farm Bureau, Mark was a professional educator in the Oklahoma public school system for 11 years. Along with his teaching experience, Mark also had administrative experience serving as a federal programs director for Plainview Public Schools in Ardmore, OK.

Mark is a graduate of East Central University in Ada, OK and holds a bachelor's degree in History and master's in Secondary Administration. Mark and his wife Stacy reside in Edmond, OK where they are active in their community and have two children, Londyn and Steele.

20+ years

Named Top Lobbyist by Austin Business Journal Girl Scouts' "Woman of Distinction" AwardAustin Business Journal's "Profiles in Power" "Austin Under 40" AwardGlossy 8 list by Austin American-Statesman Austin Fashion Week's "Style Setter" Award

Lobbyist, political fundraiser and national political commentator Andrea McWilliams is a skilled strategist, combining grace with grit and patience with persuasion. Known for her meticulous attention to detail, McWilliams brings her unique expertise to every issue she engages drawing on her time in both the private and public sector. The national media, including FOX News, the Wall Street Journal, Newsweek, USA Today, CNN, BBC, and NPR have all reported on McWilliams' role as a leading fundraiser and political strategist. Named Texas Businesswoman of the Year by the Women's Chamber of Commerce of Texas, she was recognized as "consigliere to the powerful and political." McWilliams was the only lobbyist profiled together with the Governor, Lieutenant Governor and Speaker of the House in Texas Monthly's "POWER" feature highlighting the most powerful people at the Texas Capitol.

McWilliams is also a proud recipient of both the Austin Business Journal's "Profiles in Power" and the Girl Scouts' "Woman of Distinction" Award. She has received the "Austin Under 40" Award, and the Austin American-Statesman honored her as a member of its Glossy 8 list. Last year, McWilliams received Austin Fashion Week's "Style Setter" Award, recognizing her work and dedication on behalf of local nonprofit organizations.

A Chief of Staff at the Texas Capitol at just 21, McWilliams went on to hone her private sector grassroots skills at the iconic public relations company Public Strategies, Inc. Today, she is the President of McWilliams Governmental Affairs (MGA), a full-service firm of governmental affairs consultants, respected and trusted at the local, state and national level.

Family and Community:

Raised just steps from the Texas Capitol on Congress Avenue, McWilliams now resides in Old Enfield, raising three children in a historic home.

She is a well-known philanthropist, community volunteer and proud native of Austin, named on many occasions to the "All-Stars" of Austin American Statesman Fortunate 500 list. McWilliams is honored to serve on the Board of St. David's Medical Center, one of the largest health systems in Texas and the third-largest private employer in the Austin area. Last year, McWilliams was also appointed to the Board of the Save Historic Muny District whose charge is to preserve 141-acres of green space in Central Austin that includes Lions Municipal Golf Course ("Muny"), widely recognized as the first public golf course in the South to become integrated and listed in the National Register of Historic Places. Additionally, she enjoys her time on the Pease Park Capital Campaign Committee whose efforts will transform this important landmark for future generations in her hometown.

McWilliams currently serves on the PAC Board of RYAN, an award-winning global tax services firm based in Dallas. And, she has also served as a board member for many other non-profit organizations, including Texas Lyceum, Mexi-Arte Museum, Arthouse, Rise Across Texas Challenge, HeartGift, Austin Children in Crisis, Pioneer Farms and the Waterloo President's Council. She is an inaugural member of the Long Center's Notable Women and a long-time member of the St. David's Hospital Toast of the Town Committee. True to her roots, McWilliams is a proud past President of Inherit Austin, an organization founded to preserve Austin's architectural and cultural past by not only protecting the city's important landmarks and historical fabric, but safeguarding its unique character as well.

McWilliams has represented the Susan G. Komen Breast Cancer Foundation and KillCancer, two nonprofit groups committed to cancer prevention. She was also honorary chair for the Mamma Jamma Ride, which since its inaugural ride, has raised millions of dollars for non-profits serving thousands of Central Texans coping with breast cancer. In recognition, the Seton Breast Cancer Center in Austin named their screening room after McWilliams. In 2018, McWilliams additionally served as the honorary chair of the Celebration of Life, the most significant fundraising event for breast cancer in Austin, Texas.

In partnership with the Austin Business Journal, McWilliams enjoys volunteering as a business mentor to other young women in Central Texas. Aside from her continued board service with Preservation Austin, Ballet Austin and the Pease Park Conservancy, she also served as a member of the Girl Scouts Centennial Celebration Committee and chaired Austin's largest charity event, the Ballet Fete, benefiting Ballet Austin. McWilliams additionally co-chaired the Marathon Kids gala and was a chair of the University of Texas Blanton Museum of Art Gala. In support of the Center for Child Protection, McWilliams was a celebrity dancer for Austin's inaugural "Dancing With The Stars" fundraising event. She is a proud past member of the Texas Emerald Ball Committee, whose endeavors support the American Ireland Fund.

During the 86th Legislative Session, McWilliams also served on the Governor For A Day Committee.

6-10 years

A native of Michigan's rugged, beautiful Upper Peninsula, Matt joined Karoub Associates in 2011 after serving eight years on legislative staff for a former State Senator and Senate Minority Leader. He holds degrees from Western Michigan University, Gogebic Community College, and is a 2005 Fellow of the Michigan Political Leadership Program (MPLP) at Michigan State University.

An avid golfer, musician, and NASCAR enthusiast, Matt and his wife Rebecca reside with their son in Okemos.

Lauren McCay serves as a Senior Associate in Cornerstone's Oklahoma City office, assisting the firm's government relations team with client service activities including conducting research and providing updates on state legislative matters and diverse regulatory issues.

Lauren comes to Cornerstone from Lilly & Company where she led the firm's highly successful fundraising effort in Oklahoma as an Account Executive. She was responsible for executing numerous fundraisers and meeting fundraising goals for current federal legislators and those seeking office in the U.S House and Senate.

Before joining Lilly & Company, Lauren served as a Congressional Intern for Representative Stephanie Bice (R-OK) where she managed and answered constituent inquiries while also supporting staff by taking initiative on various district office projects and case work.

Lauren received her Bachelor of Science in political science from the University of Oklahoma.

Prior to joining JohnsonPossKirby in 2017, Luke served Capitol Hill in the Tennessee General Assembly, the Department of Finance and Administration, and the Office of Governor Bill Haslam.

Luke began working in politics in 2009 for Representative Pat Marsh and then joined Governor Haslam's office in February of 2011. During his tenure there, Luke helped shepherd the Governor's signature legislation through the legislative process. These initiatives include teacher tenure reform, tort reform, the TEAM Act, the Prescription Safety Act, and TN Promise to name a few. In 2015, Luke left the Governor's office to join Commissioner Larry Martin and the Department of Finance and Administration.In his role as Legislative Liaison, Luke was responsible for communicating with the legislature on behalf of Commissioner Martin and the divisions within the department. Luke worked with the Commissioner, Strategy Technology Solutions (STS), Benefits Administration, and the Division of Budget to achieve the department's goals and facilitate a strong working relationship between the department and General Assembly.

In addition to his work with the F&A divisions, Luke played a role in preparing the Governor's Budget and advocating for its approval in the General Assembly. These budgets included the largest tax cut in state history, two debt free budgets, and significant investments in recruiting jobs.Luke and his wife, Katie, live in Franklin with their two daughters, Ryall and Mackie.

Nathan Poss has over 20 years of experience in both the public and private sector. His focus is to provide clients with a comprehensive range of services, including: legislative drafting and tracking, policy development, governmental lobbying (at all levels), strategy development, and business procurement.



Prior to forming JohnsonPossKirby Government Relations in 2007, Nathan served as a principal at Smith Johnson & Carr, focusing on lobbying efforts for the firm's business clients and taking the lead on procurement strategies.



Before entering into the lobbying world, Nathan served seven years on Capitol Hill as a legislative assistant to then Speaker Pro Tempore of the Senate, State Senator Bob Rochelle. During his tenure with the state, he played an important part in writing and developing legislation, organizing political strategies, building coalitions, and monitoring legislation for committees. Poss has also been actively involved in numerous successful political campaigns.



Poss has built a reputation of hard work and respect among those on Capitol Hill. Working with major players in the state's business community, he has served as a key strategist on major environmental and tax issues. Nathan has garnered a reputation for producing tangible results through well-executed and intricate lobbying efforts designed to achieve long lasting success for his clients.

In addition to his lobbying successes, Nathan has extensive experience with the procurement process in Tennessee. Recently recognized by the TN Report for being known as an "expert in the state's purchasing practices," Nathan has cultivated the necessary connections with top-level decision makers throughout all areas of state government. Throughout his practice he has developed a strong grasp of the state's purchasing processes, and has successfully secured multi-million dollar purchases for numerous clients.

Nathan is a graduate of Cumberland University with a B.A. in Social Science. A native Tennessean, Nathan grew up in Lebanon and currently lives in Green Hills with his wife Molly.