Martynas acts for football players, clubs, agents and federations — in contract disputes, transfers, disciplinary matters and arbitration before FIFA and the Court of Arbitration for Sport — and for businesses in commercial litigation and arbitration. He has practised for more than 15 years; before founding LEXTRA he worked in-house at a professional football club and spent a decade at one of the Baltic region's leading law firms.

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He is the sole author of “Sports Law in Lithuania” (Kluwer Law International, 2017; updated 5th edition will be released in fall 2026) — the reference monograph on the subject. He served on the Legal Advisory Panel of the European Club Association (ECA), acts as counsel to the Lithuanian Professional Football Players' Association in its work with FIFPRO, and was named a Rising Star in litigation by The Legal 500 EMEA.

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He is also a FIFA licensed football agent — one of the few practitioners in the region who can both negotiate the deal and litigate it. He is a regular participant and speaker at international sports law conferences (Lausanne, Rome, Kyiv — including on esports arbitration), holds an Executive Master in International Sports Law from ISDE (Madrid), and works in Lithuanian, English, and Italian.