Allgood began work as soon as she was able, apprenticed to a French polisher near her mother's workplace who sold high quality antique and modern furniture from their warerooms at 19 and 20 Bachelors Walk, Dublin, and who also operated as cabinet-makers, upholsterers, valuers, house agents, and auctioneers.

[3] Allgood joined the Irish nationalist Daughters of Ireland, where she first began to study drama under the direction of Maud Gonne and William Fay.

In 1915, Allgood was cast as the lead in J. Hartley Manners' comedy Peg o' My Heart which toured Australia and New Zealand in 1916.

She was nominated for a Best Supporting Actress Academy Award for her role as Beth Morgan in the 1941 film How Green Was My Valley.

She had memorable roles in the 1941 retelling of Dr. Jekyll and Mr. Hyde, It Happened in Flatbush (1942), Jane Eyre (1943), The Lodger (1944), The Keys of the Kingdom (1944), The Spiral Staircase (1946), The Fabulous Dorseys (1947), and the original Cheaper by the Dozen (1950).

In September 1916, Allgood married her Peg o' My Heart co-star Gerald Henson while they were touring in Melbourne.

[10][11] On 13 September 1950, at the age of 69, Allgood died of a heart attack at her home in Woodland Hills, California.