When she was 18 years old, Frothingham joined the management committee of the Montreal Protestant Orphan Asylum.

Beginning in 1890, she served on the board of governors at the Protestant Hospital for the Insane.

She was the second president of the Montreal Ladies' Educational Association, succeeding her sister-in-law Anne Molson.

[1] In 1895, she was an honoured guest at the opening ceremonies for the new medical campus at McGill.

[9] Frothingham married businessman John Henry Robinson Molson in 1873.