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

In a case in the Maryland Court of Appeals, decided 2 April 1918, in the case of The German-American Bank of Baltimore vs. Louisa May and Mary C. Kopp, it was stated that John Mahr died on 27 September 1917.
[Maryland Reports: Cases Adjudged in the Court of Appeals of Maryland, Volume 132, p. 423.]