He chose instead to move to Kaunas, which had become the temporary capital of the recently re-established independent Republic of Lithuania.
He was one of szlachta members loyal to the heritage of Grand Duchy of Lithuania, referred to as Krajowcy. He gave the first lecture on Lithuania in Paris in 1905, together with another Lithuanian activist, Tadas Ivanauskas. In 1908 he wrote Lithuania: Study of national revival (Litwa: studium odrodzenia narodowego).
In 1920, he declined an offer to become the prime minister of the Republic of Central Lithuania.
As an international lawyer, in 1932 he represented Lithuania at the Permanent Court of International Justice regarding the Klaipėda Directorate. The court found in Lithuania"s favor that Otto Böttcher had violated the Statute of the Klaipėda Region.
Römer wrote important works on Lithuanian history and on law, such as the 1908 book Studija apie lietuvių tautos atgimimą and the 1928 book Die Verfassungsreform Litauens im Jahre 1928. He is considered to be one of the most prominent Lithuanian jurists, the progenitor, first lector and one of the most prominent authors of interwar Lithuanian constitutional law.
While most of his writings on lithuanian law were written in Lithuanian and his signature on lithuanian documents and letters was Mykolas Römeris, (sometimes also credited as Mykolas Rėmeris or Mykolas Riomeris), he continued to write his diary in Polish and use the original german form of his name (Michael von Römer) for his law writtings in german.
The Law University of Lithuania in Vilnius was renamed to the Mykolas Romeris University in 2004.
Later Römer became a member of Piłsudski"s Polish legions, but broke with the Polish side. He was a notable figure in the interwar period, and was a member of the Lithuanian Supreme Court (1921–1928), a professor at the University of Lithuania (in 1930 renamed to Vytautas Magnus University) (1922–1940), Vilnius University (1940–1945) and the rector of the University of Lithuania for three terms (1927–1928, 1933-1936 and 1936–1939).