Henry Kopp married Mary K. Mahr on 28 June 1874. Both lived in Baltimore. He was 23, white, single, a cigar-maker; she was 20, white, single. The marriage license was issued 16 June 1874 and filed on 18 November 1874. The minister was L. D. Maier. [Hall of Records, Annapolis, Maryland, microfilm CR-1674, with reference to BK 4-1-F-R-1865–75, folio 264.]

1880 U.S. Census, Baltimore City, Maryland
1st Ward, 3rd Precinct
Page 90
314 Alice Anna Street
Dwelling 145, Household 242
Kopp, Henry, white, male, age 29, married, occupation Cigar Maker, born in Maryland, both parents born in Maryland
Kopp, Mary, white, female, age 25, married, occupation Keeps House, born in Maryland, both parents born in Germany

1900 U.S. Census, Baltimore City, Maryland:
5th Ward, Enumeration District 62, Sheet 1
1714 Gough Street
Dwelling 5, Household 7
Mahr, John, head, white, male, born January 1853, age 47, single, born in Maryland, both parents born in Germany, occupation huckster, 0 months unemployed, can read and write, can speak English, owns, free, house
Kopp, Mary, sister, white, female, born July 1846, age 53, widowed, 0 children [may say 3], born in Maryland, both parents born in Germany, can read and write, can speak English

1910 U.S. Census, Baltimore City, Maryland
7th Ward, Supervisor’s District 3, Enumeration District 21, Sheet 12B
1714 Gough Street
Dwelling 177, Family 203
Mahr, John, head, male, white, 65, single, born in Maryland, both parents born in Germany and spoke German, speaks English, produce dealer for market, works on own account, can read and write, owns house
Koff, Mary C., sister, female, white, 55, widowed, born in Maryland, both parents born in Germany and spoke German, speaks English, no occupation, can read and write

The 1921 Baltimore City Directory lists on page 1246:
Kopp, Mary C (wid Henry) h 2824 E Balto