Thomas Allen's practice focuses on complex civil litigation at the trial and appellate levels in state and federal courts. His practice includes trial briefing, direct appeals, interlocutory appeals, restricted appeals, motions for rehearing, and mandamus proceedings. Thomas has extensive experience representing clients before the Supreme Court of Texas, intermediate Texas courts of appeals, and federal courts of appeals and has presented oral argument to many courts of appeals.

Prior to joining Jones Day in 2013, Thomas achieved positive outcomes for clients in numerous matters. He drafted briefs and presented oral argument in obtaining affirmance of summary judgments in two companion cases involving the sales of oil and gas royalty interests; after requesting responses to the plaintiffs' petitions for review, the Supreme Court of Texas denied review. He also was a member of appellate teams that achieved reversal in the Supreme Court of Texas of the large majority of damages awarded against a major international law firm in a legal malpractice action; achieved reversal in the Dallas Court of Appeals of the trial court's refusal to dismiss medical malpractice lawsuit for failure to timely file an expert report; and obtained dismissal of bankruptcy appeal for lack of standing in the U.S. Court of Appeals for the Fifth Circuit.

Thomas is a member of the State Bar of Texas (Appellate Section; co-chair, Texas Appellate Hall of Fame Committee), the Dallas Bar Association, the Dallas Zoological Society (board of directors), and the Dallas Committee on Foreign Relations. He has written and spoken on the appellate process.