Based on the census data she appears to have been born in Alabama about 1824 not 1812 as many sources mention. Her oldest child was 7 in the 1850 census and he was born in Alabama. So, she likely married WIlliam there. No marriage record found.
Based on the census data, she had at least 8 children
Joseph Warren b 4/15/1842 Alabama - his biography is at d 7/13/1904
Mary E b 1/15/1847 Mississippi, married William L Hemingway. She died 12/3/1935
Ann Elizabeth b 1848 Mississippi - last found in the 1870 census age 20 and single living at home with home.
Jeannie b 1855 Mississippi - who may have died between 1860 and 1870
Sallie b 1857 Mississippi, married Allison Topp and she died 3/31/1887 
Henry Pinkney b 1/23/1861 Mississippi d 7/24/1941 and is buried at Arlington Cemetery
William H born 1863 Mississippi - last found in 1870 census
The 1850 census shows her age as 26 - she is in the household of Thomas Peete with her husband and 3 children. Their relationship with the Peete family is unknown. Perhaps the census taker just numbered the household incorrectly.
The 1860 census shows her age as 36 - she is listed as L McCain with WA McCain and 7 children are in the household with ages from less than a year to age 17. All children were born in Mississippi except the oldest JW who was born in Alabama.
The 1870 census shows her age as 45 - she is listed as M Lor Mccain with 4 children ages 7 to 20 in the household.
The 1880 census shows her age as 56 - she is living with her son John and his family and her two unmarried children Sallie and Willie.
Although no tombstone is shown, findagraveshows her birth as 12/2/1812. She is buried in
Waverly Plantation Teoc, Carroll County, Mississippi, USA
↑ Perrin etal, Kentucky, a History of the State, Southern Historical Press, 1887. Page 818.
"United States Census, 1850," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M4LN-8J9 : 12 April 2016), Louisa Mc Cain in household of Thomas Peete, Tallahatchie county, Tallahatchie, Mississippi, United States; citing family 361, NARA microfilm publication M432 (Washington, D.C.: National Archives and Records Administration, n.d.).
"United States Census, 1870," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MFS8-2T7 : 12 April 2016), M Lor Mccain, Mississippi, United States; citing p. 4, family 27, NARA microfilm publication M593 (Washington D.C.: National Archives and Records Administration, n.d.); FHL microfilm 552,222.
"United States Census, 1880," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M4G9-V1M : 12 August 2017), Mary L Mccain in household of John Mccain, Beat 2, Carroll, Mississippi, United States; citing enumeration district ED 19, sheet 31A, NARA microfilm publication T9 (Washington D.C.: National Archives and Records Administration, n.d.), roll 0642; FHL microfilm 1,254,642.