She was first elected to Dáil Éireann as a Cumann na nGaedheal Teachta Dála (TD) at the 1932 general election for the Leitrim–Sligo constituency. Her husband Patrick Reynolds had been a TD for the same constituency in the 6th Dáil, but was fatally shot during the 1932 general election campaign. The election was postponed, and Mary won the seat, serving in the Dáil for 25 years.
She lost her seat at the 1933 general election but was elected for the Leitrim constituency at the 1937 general election as a Fine Gael TD. She represented the Sligo–Leitrim constituency from 1948 until her retirement at the 1961 general election.
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