According to the 1850 Territorial census, John was born in 1840; but the 1870 Federal census says that he was born in 1844. He is the son of Michel St. Cyr and Julia Angelique, and his half-brother Moses was born in the same year, a fact that may have been covered up in 1850, since their father had two wives at once.
The 1870 census states that he was "Capt. of police."
In 1870 (census), he was living with his half-brother on Moses' farm in McPherson, Blue Earth, Minnesota.
1870 Federal Census, McPherson, Blue Earth, Minnesota; Roll: T132_1; Page: 462; National Archives Microfilm Publication M432, 1009 rolls; Records of the Bureau of the Census, Record Group 29; National Archives, Washington, D.C.