She married barrister and writer Maurice Henry Hewlett on 3 January 1888 in St Peter's Church, Vauxhall, where her father was the incumbent.

The family acquired a car and Hilda learned to drive, and was fined for speeding in May and June 1905 but was considered proficient behind the wheel.

She participated in automobile rallies[3] and in 1906 was the passenger/mechanic for Muriel Hind, the only female driver in a motorcar event from Lands End to John O’Groats.

Later that year, after adopting the pseudonym "Grace Bird", she travelled to the airfield at Mourmelon-le-Grand, France, to study aeronautics.

In the summer of 1910 she and Blondeau opened the first flying school in the United Kingdom at the Brooklands motor-racing circuit at Weybridge, Surrey.

Thirteen pupils graduated from the school in the year and a half it operated and, with a remarkable safety record for the time, there were no accidents.

Pilots were to fly for only the minimum time to qualify for prizes, and the aeroplanes were locked away to prevent inspection by the public.

[5] On 29 August 1911, at Brooklands, Hilda Hewlett became the first woman in the UK to earn a pilot's licence when she received certificate No.122 from the Royal Aero Club after completing the test in her biplane.

In January 1939, at the opening of a new aerodrome in Tauranga, Frederick Jones, New Zealand's then Minister of Defence, named a nearby road after Hilda Hewlett and her son Francis, in recognition of their services to aviation.

A staff photograph and autograph book from the wings section of the Hewlett and Blondeau factory, dated to the First World War, was donated to the Women's Engineering Society's archives in 2022.

[12] In October 2022, Air Chief Marshal Sir Mike Wigstone opened the RAF's Hilda B Hewlett Centre for Innovation, named in her honour.

The facility specialises in 3D printing and scanning equipment, part of the Royal Air Force’s first steps into advanced component manufacturing.