McArthur was born in Denver, Colorado and, having begun work as a professional pianist, studied piano at the Juilliard School and moved to New York as a music teacher and organist.
He applied for a post to play for Flagstad soon after her debut at the Metropolitan Opera (which took place on 2 February 1935) and she chose him for her accompanist for her tour of that year.
In 1941, he became possibly the first American-born conductor to lead a work at the Metropolitan Opera House, conducting Flagstad and Lauritz Melchior in Tristan und Isolde.
He remained her conductor right through to her retirement in 1955, persuading her to make final appearances in Wagnerian scenes at her farewell concert at Carnegie Hall on 20 March 1955, for the benefit of the Symphony of the Air.
[2] This gossip has been disproved by a number of sources, including Cosmé McMoon's living family and the New York Times's obituaries for both men.