He was educated at Erasmus Hall High School in New York City, before enrolling at the University of North Carolina from where he graduated with a Bachelor of Arts in 1915.

Mallett was ordained deacon in May 1918 and priest in December 1918 by the Bishop of North Carolina Joseph Blount Cheshire.

[3] On July 26, 1944, Mallett was elected Bishop of Northern Indiana on the first ballot, during a special convention.

[4] He was consecrated on October 25, 1944, in St James' Church, South Bend, Indiana.

Mallett was episcopal visitor to St. Gregory's Abbey, Three Rivers, Michigan and served as Superior-General of the American Branch of the Confraternity of the Blessed Sacrament from 1946 to 1965.