In 1907 he went to Britain on business and stayed, joining Sperling & Co Ltd, merchant bankers, of Moorgate Street, London, in 1908 and later becoming a senior partner.

He left Sperlings at this time and organised his own company, E. Mackay Edgar Ltd, with offices in London, Montreal and New York City.

This ended when he retired due to ill-health, but in 1928 he returned with a new company, Beeston's Trust Ltd, which folded during the Depression and in 1931 he again went into receivership.

Mackay Edgar's only son, John, was killed in a car accident in July 1925 and the baronetcy therefore became extinct upon his death.

Mackay Edgar and his wife, Ethel Beatrice (née Pinder), a fellow native of Montreal whom he married in 1902, are buried in the cemetery in Chalfont St Peter, Buckinghamshire.