He studied philosophy and medicine in Prague (postgraduate in Vienna).He promoted in 1886.
He became a professor in 1890. He became a chairman of the Physiology Institute of the Charkes University in 1895, continuing in that capacity until 1928. In 1914 and 1920, he was appointed rector of the Charles University for one year.
He was many times appointed dean of Faculty of Medicine.
Honorary doctorate Mayo clinic. During the war and after the independence of Czechoslovakia from Austria-Hungary in 1918, Mareš became active in the Czechoslovak nationalist movement.
In 1920, he was elected to the Senate, until the next election of 1925. He was also a contributor to Vlajka.
At that time he became influenced by fascism, much to the dismay of the National Democratic leadership. In 1934, he was part of the leadership of the National Front (Národní fronta), later National Unity (Národní sjednocení).
He was rector of the Charles University in 1920-1921, and member of the National Democrats. He was an active member of the National Democrats, and became a member of the Revolutionary National Assembly of Czechoslovakia in 1918.