Kazys Grinius was born in the village of Selema-Buda, the Sasnava Rural District, the then Marijampole District, on December 17, 1866, in the family of a bright peasant who owned a farm of 17 hectares. The family had a home library, quite extensive for the then period, and the children from their childhood could get acquainted with the works of most Lithuanian writers (Valancius, Ivinskas, etc.). Also favoured was Ausra (The Dawn), which was smuggled from Prussia.
Kazys Grinius, after graduating from the Marijampole gymnasium, in 1887 entered Moscow University to study medicine. Here he got involved in the activity of the secret Lithuanian Student Association under Petras Leonas, started attending lectures on the Lithuanian language by Prof. Fortunatov, collaborated with the Lithuanian press published in Prussia and America. In 1888 he, together with V. Kudirka, P. Leonas, P. Kriaciunas, J. Staugaitis, J.Bagdonas, V. Palukaitis and other broad-minded Lithuanians, organized the first Congress of the Lithuanian Democrats in Marijampole, was the founder of the Varpas (The Bell) newspaper and somewhat later set up Ukininkas (The Farmer), was a most active contributor to these newspapers, edited Lietuvos ukininko kalendorius (The Lithuanian Farmer's Calendar). In 1889, for participation in the Moscow students' riots K. Grinius was jailed in the Butyrki prison. From the funds, collected at lotteries, K. Grinius in 1891 established the foundation for the publishing of Lithuanian books. In 1893, after receiving a doctor's diploma, he worked as a ship's physician in the Caspian Navy; afterwards, having returned to Lithuania, he settled in Marijampole and began to earn his living as a practicing physician, was elected to the Varpininkai Committee - for publishing Varpas and Ukininkas. In 1895, K. Grinius, 5. Matulaitis and J. Bagdonas entered into contacts with the Social Democrats from Vilnius A. Domasevicius, A. Moravskis, etc., started collaborating in their press, supplied the Social Democrats of Vilnius and Kaunas with proclamations and other publications. In tile autumn of 1896, having moved to K. Naumiestis, he associated with V. Kudirka, was concerned with Varpas. In 1902, together with J. Vileisis, J. Bortkevicius and J. Saulys he prepared a draft programme of the Democratic Party of Lithuania, in 1904 organized a public demonstration of Lithuanians at the grave of Petras Arminas in Marijampole. K. Grinius' house became the important abode of the Lithuanian Revolutionary Democrats and Social Democrats in Suvalkija. With K. Grinius' concern a Lithuanian theatre was established in Marijampole, in which on May 28, 1905 for the first time in public, with the participation of the choir, the performance Amerika pirtyje (America in the Bath-house) was shown. K. Grinius' efforts resulted in the establishment of the first open school with Lithuanian as a language of instruction in Marijampole in 1905. But soon, under the pressure of the Tsarist officials, he and his family had to leave Marijampole and moved to Vilnius. Here he got actively involved in journalistic activity, wrote much for Lietuvos ukininkas (The Lithuanian Farmer) and Vilniaus zinios (The Vilnius News), issued brochures of political character. Having come back to Marijampole next year, he participated in the activity of the educational society Sviesa (The Light). For his active participation in Lithuanianism more than once he was confined in a jail in Marijampole by gendermes, unless at the end of 1908 he again was exiled to Vilnius, where he lived until 1910. Here he together with A. Rimka edited Lietuvos ukininkas, some time later Lietuvos zinios (News of Lithuania) with G. Petkevicaite. K. Grinius with F. Bortkeviciene established a company for publishing the Lithuanian press. In 1911 in Marijampole he organized the first Lithuanian agricultural exhibition. As K. Grinius did not allow a poster with Russia's anthem to be hung at the exhibition, he was incarcerated in the Marijampole prison for 1.5 months.
At the outbreak of World War I he retreated to Russia. In 1918 in Kislovodsk his wife Joana and daughter Grazina were killed. At the end of 1919 K. Grinius returned to Lithuania via Turkey and France and joined the work for the defence and consolidation of Lithuania's independence. He was elected to the Constituent Seimas, served as deputy Prime Minister (June 19, 1920-February 1, 1922), wrote much for the press, edited Lietuvos zinios. Since the summer of 1922 K. Grinius started heading the medicine and sanitary departament at the Kaunas municipality.
In the summer of 1926, after the left parties won elections to the Seimas, Dr. Kazys Grinius was elected President of the Republic. But on December 17, 1926 he was forced to resign from his post as President and returned to old duties in the municipal administration. With K. Grinius' efforts several organizations for care of mothers and children were established. In the years of the German occupation he supported the Lithuanian anti-Nazi resistance, protested against the massacre of Jews, and was exiled by the Germans from Kaunas to Azuolu Buda. The Red Army approaching Lithuania, K. Grinius and his family with drew to Germany. In 1945-1946 he found shelter at the displaced persons' camp in Hanau.
At the beginning of 1947 K. Grinius emigrated to the USA, to Philadelphia, after a year settled in Chicago. Here he wrote memoirs, articles for the press, began to prepare the dictionary of medical terms, took part in the activity of Lithuanians, primarily of the Peasant Populist Union. President Kazys Grinius died in Chicago on June 4, 1950.