The only May/Mary Leahy I found in the 1880 Census was b. 1875 to John and Hannah; Mary b. 1871 to John and Ellen; and a 'Katie' b. 1874 to Dennis and Mary.
These two death records are for May Hancock - both with correct birth date - but different death dates - one lists mothers maiden name as Nagle. No marriage record found. On Charles draft record he is a patient at Mendocino State Hosp. Not much help.
U S Social Security Death Index
Citing this Record "United States Social Security Death Index," index, FamilySearch (https://familysearch.org/pal:/MM9.1.1/JBQF-835
May A Hancock
Birth 11 April 1879
Social Security Number: 546-80-3770
State: California
Last Place of Residence: San Mateo, California
Previous Residence Postal Code: 94401
Event Date: 15 June 1967
Age: 88
California, Death Index, 1940-1997
Citing this Record "California, Death Index, 1940-1997," index, FamilySearch (https://familysearch.org/pal:/MM9.1.1/VP4T-PRR
May K Hancock
Death 30 Jun 1967 Santa Clara, CA
Birth Date: 11 Apr 1879 California
Mother's Name: Nagle
U S Census, 1910 San Francisco Assembly District 42, San Francisco, CA
SELF Charles R Hancock M 29y IN/IN/IL
WIFE Mary C Hancock F 28y CA/IRE/IRE
SON Charles A Hancock M 4y CA
SON Vincent A Hancock M 2y CA
U S Census, 1920 Precinct 14 Assembly 31 San Francisco, San Francisco, CA
Citing this Record "United States Census, 1920," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/MHW5-PRP
SELF Charles R Hancock M 38y IN
WIFE May C Hancock F 40y CA
SON Vincent Hancock M 12y CA
SON Charles W Hancock M 14y CA
SON Joseph Hancock M 7y CA
SON Francis Hancock M 6y CA
SON George Hancock M 2y7m CA
DAU Madeline Hancock F 9y CA
DAU Mary B Hancock F 4y5m CA
U S Census, 1930 San Francisco (Districts 251-409), San Francisco, CA
Citing this Record "United States Census, 1930," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/XCN7-HH8
Head Mae Hancock F 45 CA/IRE/IRE
Son Vincent Hancock M 22 CA
Son Thomas J Hancock M 19 CA (? Joseph)
Son Frank Hancock M 15 CA
Son George Hancock M 12 CA
US Census, 1940 Assembly District 22, San Francisco, San Francisco City, CA
Citing this Record "United States Census, 1940," index and images, FamilySearch (https://familysearch.org/pal:/MM9.1.1/K98V-N2D
Head Roy Barsotti M 34 CA
Wife Madaline Barsotti F 29 CA
Dau Louise Barsotti F 8 CA
Dau Valerie Barsotti F 5 CA
Bro-in-law George Hancock M 23 CA
Moth-in-law May Hancock F 60 CA Single
World War I Draft Registration Cards, 1917-1918
https://familysearch.org/pal:/MM9.3.1/TH-1961-26182-65304-34?cc=1968530&wc=M9SG-SQT:1109081054
Charles Ransome Hancock
Draft Registration Sept. 1918
Registration Place: Mendocino State Hospital, San Francisco City no 12, CA
Male Male Married
Birth 25 Feb 1881 United States
Nearest Relative: Mrs. Mary A Hancock, 1305 Greenwich St., San Francisco
Affiliate Publication Number: M1509
GS Film number: 1544262
Digital Folder Number: 005241080
Image Number: 01584
