Alkio's parents were Juho and Maria (nee. Jakku) Filander. He married teacher Anna Augusta Falenius in 1896.
Initially Santeri Alkio was active in the Nuorsuomalainen Puolue (Young Finnish Party), but in the end decided it was too liberal for the farming population; urbanized parties did not, in his estimation, pay enough attention to the causes that were most important to farmers. To keep the agrarian folk from becoming ensnared by socialism, he founded the Etelä-Pohjanmaan Nuorsuomalainen Maalaisliitto, which he later fused into the less ideological Maalaisväestön liittoon (Centre Party of Finland). Alkio became the chief ideologue of the Maalaisliitto, and is still considered the father of the party in spirit. The party still refers to alkioish tendencies in some of its factions.
Alkio was a member of the Finnish House of Representatives from 1907 - 1908 and again from 1914 - 1922. He was vice-chairman of the Eduskunnan in 1917 and 1918, and minister of social affairs from 1919 - 1920. He was the minister of social affairs of the Vennola government from (August 15, 1919 - March 15, 1920). An ardent temperance-movement activist, he participated in drafting the Finnish Prohibition and also was the minister responsible for the confirmation of president K. J. Ståhlberg.
Alkio was an extremely prolific author. He founded the newspaper Ilkka and was its editor through the years 1906 - 1930.
His likeness graced a Finnish stamp on July 17, 1992.
Alkio was a fervent spokesman for democracy and Finnish national independence. He led the youth association movement, which above all wanted to defend the values of rural life and foster temperance and healthy living, a desire the movement held in common with the coeval Christian revivalist and labor movements.
As a nationalist, Alkio supported the independent senate of Svinhuvud. During the summer of 1917 he had supported usurping the highest power in the land from Russia via the Power of Government Act (Lex Tulenheimo) while the parties on the right still opposed it.
Alkio thought the red revolt supported by Russian soldiers was an attempt to return Finland to Soviet Russia: "It [the revolt] is meant to set Finnish independence at nought." ("Sen [kapinan] tarkoituksena on tehdä tyhjäksi Suomen itsenäisyys.")
He was also a pacifist.