Katherine Alexander Duer Mackay (1878–1930) was an American suffragist, socialite and writer from New York city. She was the founder of the Equal Franchise Society. Her involvement with the woman's suffrage movement "encouraged other wealthy women to follow her lead and become involved." She was also the first female member of the Roslyn Union Free School District's school board in Roslyn, New York.
Katherine Duer was born in New York City in 1878. She was a direct descendant of Lady Kitty Duer, daughter of Lord Stirling. She married Clarence H. Mackay in 1898. Mackay was well known in connection with Harbor Hill where she was involved with philanthropy and education. She lived in Roslyn from 1898 to 1910. In 1899, Mackay refurbished Roslyn's public library, the William Cullen Bryant Library. She installed new carpet, replaced books and hired two librarians. Mackay would invite people, and children, to the house for various functions. Once, the Tr