Zac is a trial lawyer who represents clients in complex, high-stakes commercial disputes. His practice includes representing private equity sponsors and portfolio companies, public companies, and individuals in matters involving breach of contract, fraud and business torts, environmental and mass-tort claims, and labor and employment disputes. He has successfully tried cases in federal and state courts across the country, as well as in arbitration before the American Arbitration Association and the National Football League.
Before co-founding Hayden Ryan & Ciullo, Zac was a litigation partner at Kirkland & Ellis in Chicago. There, he represented clients across a wide range of industries—including pharmaceutical, automotive, railroad, construction, chemical, oil and gas, sports and entertainment, beauty and cosmetics, and cryptocurrency—handling matters at all stages of litigation, from initial pleadings through trial and appeal.
Earlier in his career, Zac clerked for the Honorable Michael S. Kanne of the United States Court of Appeals for the Seventh Circuit where he managed and advised on numerous civil and criminal appellate cases.
Zac is licensed in Illinois and Ohio; he received the highest score on the Ohio Bar Examination. He is currently a member of the International Association of Defense Counsel and a board member of the Football Alumni of Michigan Teammate Assistance Program.
Zac maintains an active pro bono practice focusing on criminal matters for underserved and disadvantaged individuals.
experience
Representative Matters
Trial counsel for a Fortune 100 pharmaceutical company in nationwide litigation concerning the marketing and sale of prescription opioid medications; obtained a complete defense verdict at trial in California and favorable resolutions in multiple related cases.
Trial counsel for a railcar owner in a contribution action arising from the 2023 East Palestine, Ohio train derailment; obtained a total defense verdict following trial in the Northern District of Ohio.
Trial counsel for a minority owner in arbitration before the National Football League involving a team ownership dispute; matter resolved favorably before decision.
Trial counsel for a Fortune 500 healthcare provider in arbitration concerning revenue-cycle management services; opposing party filed for bankruptcy prior to post-trial briefing.
Trial counsel for an oil refiner in a post-closing purchase price dispute; obtained a favorable result following trial and arbitration proceedings in Delaware.
Trial counsel for a Fortune 500 paper manufacturer in a CERCLA action regarding PCB contamination of the Kalamazoo River; case proceeded to verdict at trial in the Western District of Michigan and was ultimately reversed on appeal in our client’s favor.
Litigation counsel for a Fortune 150 pharmaceutical company; assisted in securing dismissal with prejudice of a nationwide class action alleging multiple product-liability theories.
Litigation counsel for a Fortune 50 automotive OEM in litigation involving allegations of corruption and bribery related to the collective-bargaining process and bribery of UAW officials.
Litigation counsel for a beauty and cosmetics company in nationwide litigation concerning the manufacture, marketing, and sale of hair-relaxer products.
Litigation counsel for a cryptocurrency company in Chapter 11 proceedings in the Southern District of New York.
Lead trial counsel for a domestic violence survivor in criminal resentencing proceedings; secured client’s early release from prison.
Lead appellate counsel in a criminal matter challenging client’s mandatory life sentence in the Supreme Court of the United States and the United States Court of Appeals for the Seventh Circuit.
Credentials
Kirkland & Ellis LLP
- Partner, 2020–2026
- Litigation Associate, 2017–2020
United States Court of Appeals for the Seventh Circuit
Judicial Law Clerk to the Honorable Michael S. Kanne, 2016–2017
Baker & Hostetler LLP
- Litigation Associate, 2014–2016
- Summer Associate, 2013
Dickinson Wright PLLC
- Summer Associate, 2012
Education
University of Michigan Law School, J.D., magna cum laude
- Contributing Editor, Michigan Journal of Private Equity & Venture Capital Law
- Certificate of Merit in Legal Ethics & Professional Responsibility
University of Michigan, B.A., Psychology, with honors
- Varsity Football
- Academic All-Big Ten (2010)
- Academic Judiciary Committee
- Student Athlete Advisory Council
Recognition
“Litigator of the Week” runner up by The American Lawyer Litigation Daily, May 2, 2025
Recipient of Ascend Justice’s Champion of Justice award for representation of a gender violence survivor, June 18, 2024
Member, International Association of Defense Counsel, since 2023
Representative Matters
Trial counsel for a Fortune 100 pharmaceutical company in nationwide litigation concerning the marketing and sale of prescription opioid medications; obtained a complete defense verdict at trial in California and favorable resolutions in multiple related cases.
Trial counsel for a railcar owner in a contribution action arising from the 2023 East Palestine, Ohio train derailment; obtained a total defense verdict following trial in the Northern District of Ohio.
Trial counsel for a minority owner in arbitration before the National Football League involving a team ownership dispute; matter resolved favorably before decision.
Trial counsel for a Fortune 500 healthcare provider in arbitration concerning revenue-cycle management services; opposing party filed for bankruptcy prior to post-trial briefing.
Trial counsel for an oil refiner in a post-closing purchase price dispute; obtained a favorable result following trial and arbitration proceedings in Delaware.
Trial counsel for a Fortune 500 paper manufacturer in a CERCLA action regarding PCB contamination of the Kalamazoo River; case proceeded to verdict at trial in the Western District of Michigan and was ultimately reversed on appeal in our client’s favor.
Litigation counsel for a Fortune 150 pharmaceutical company; assisted in securing dismissal with prejudice of a nationwide class action alleging multiple product-liability theories.
Litigation counsel for a Fortune 50 automotive OEM in litigation involving allegations of corruption and bribery related to the collective-bargaining process and bribery of UAW officials.
Litigation counsel for a beauty and cosmetics company in nationwide litigation concerning the manufacture, marketing, and sale of hair-relaxer products.
Litigation counsel for a cryptocurrency company in Chapter 11 proceedings in the Southern District of New York.
Lead trial counsel for a domestic violence survivor in criminal resentencing proceedings; secured client’s early release from prison.
Lead appellate counsel in a criminal matter challenging client’s mandatory life sentence in the Supreme Court of the United States and the United States Court of Appeals for the Seventh Circuit.
Credentials
Kirkland & Ellis LLP
- Partner, 2020–2026
- Litigation Associate, 2017–2020
United States Court of Appeals for the Seventh Circuit
Judicial Law Clerk to the Honorable Michael S. Kanne, 2016–2017
Baker & Hostetler LLP
- Litigation Associate, 2014–2016
- Summer Associate, 2013
Dickinson Wright PLLC
- Summer Associate, 2012
Education
University of Michigan Law School, J.D., magna cum laude
- Contributing Editor, Michigan Journal of Private Equity & Venture Capital Law
- Certificate of Merit in Legal Ethics & Professional Responsibility
University of Michigan, B.A., Psychology, with honors
- Varsity Football
- Academic All-Big Ten (2010)
- Academic Judiciary Committee
- Student Athlete Advisory Council
Recognition
“Litigator of the Week” runner up by The American Lawyer Litigation Daily, May 2, 2025
Recipient of Ascend Justice’s Champion of Justice award for representation of a gender violence survivor, June 18, 2024
Member, International Association of Defense Counsel, since 2023
