After becoming a joint partner in 1911 of the London-based legal practice, Beaumont and Son, (originally formed as a family practice by his grandfather in 1836) Major Beaumont turned the practice's focus to aviation law following an Imperial Airways accident in 1924. He was one of the three original legal advisers on the International Air Transport Association (IATA) (although it was then called the International Air Traffic Association) Legal Committee and served in this capacity from 1925 to 1946. In the early part of his career at the IATA, he was responsible for drafting the terms and conditions for passenger tickets, baggage checks and consignment notes for cargo. In 1929, Major Beaumont attended, as an observer on behalf of the IATA, a conference in Warsaw at which the Warsaw Convention for the unification of certain rules relating to international carriage by air was drafted. He was instrumental in persuading the conference members not to schedule to the Convention standard forms of tickets, baggage checks and consignment notes.