Her age is in a bit of dispute. The New York State Census of 1855 listed her as one year old, and the 1860 U.S. Federal Census said she was three years old, born in 1857. Subsequent censuses say that she was born in 1859 (1870 U.S. Federal Census), about 1860 (New York State Census of 1865), July 1862 (1900 U.S. Federal Census), and about 1863 (1910 U.S. Federal Census). She is called a variety of names in each one of these censuses: "M E Mills" (1855), "Margarett E Mills" (1860), "Margaret E Mills" (1865), "Maggie Mills" (1870), "Margaret Cosgrove" (1900), and "Margaret E Crosgrove" (1910). Using the earliest census available, her birth date is approximately 1854. Due to confusion in 1865, 1870, 1900, and 1910 censuses, she was pegged as being younger than she actually was, so her gravestone says she died at age 60, when she actually died at age 66, if we say she was born in 1854.

Margaret, or Maggie, lived out her early years (1854-1870) in Warren County New York, first in Bolton and later in Chester. Details about her life between 1870 and 1883 are not known. But it is clear that Thomas Cosgrove married her in 1883 and had a number of children. After 1895 she would adopt Mabel Hattie Packard, the daughter of Maggie's sister, Dora. Sometime between 1900 and 1910, her husband, Thomas, would die, since she is listed as a widow in the 1910 census.

When she was married to Thomas she lived in Pawtucket Ward 4, Providence, Rhode Island, definitely living there from 1900 to 1910, although her residence before that point cannot be determined. The 1910 census [https://familysearch.org/ark:/61903/1:1:MPXM-L6D] does make it clear, however, that Mabel is an adopted daughter (and single), a mother, and that she is widowed. 5 boarders were living with the family: Mary Flynn (age 26), Joseph Thompson (age 26), Josephine Marco (age 47) and a married couple named William and Alice Dalgleish, both age 28. Maggie, however, is a housekeeper.

Maggie would live out the last ten years of her life widowed. Her brother, Edward E. Mills would die in 1918 in Colorado. Two years later, in 1920, she was widowed, living in Pawtucket, Rhode Island with her adopted daughter Mabel (#181479457) who was married to Giles Thomas Whitley and had four children: Giles, Margaret, Harold, and Frances.

She would die on Feb 28, 1920 and be buried in Pottersville, NY. Nothing else is known.