Mapp was commissioned by the Salvation Army in 1888 and served in British India and Ceylon before being transferred to London.

[2] In 1929, while serving the organisation's international secretary, he participated in the first High Council of The Salvation Army, which successfully voted to remove the seriously ill General Bramwell Booth from office.

[3] Following Bramwell Booths removal from office, Mapp became Chief of the Staff of The Salvation Army under General Edward Higgins.

The five-officer panel unanimously supported Booth's decision to remove Mapp as chief of staff and found him guilty of the secret charges.

[8] Mapp claimed that he was dismissed as chief of staff because of his request to the general to retire on account of ill health.