In the early 1920s, whilst working as a bilingual secretary in Westminster, Mercedes spent her Sundays training in the Thames to fulfil a desire to become a long-distance swimmer. During her summer holiday period she travelled to Folkestone on the Kent coast to practice sea swimming.
Her initial objective was to swim across the English Channel. However, in 1923 she first set her standard as an open water swimmer when she challenged and broke the British Ladies' Record for Thames Swimming (10 hrs 45 mins), and in July 1927 she completed the 120-mile course (in stages) from London’s Westminster Bridge, down the Thames and around the headland to Folkestone.
Mercedes achieved her primary ambition on the 7th October 1927 when, on her eighth formal attempt. she became the first British woman to swim the English Channel, from France to England, in 15 hours 15 minutes. She had given up office work three months earlier in order to train full time in Folkestone, living on her savings, and her success enabled her to launch her career as a professional long distance swimmer.
Alongside open water swims, Mercedes performed a series of endurance swims in which she extended the British record from 26 hours to 47 hours. These latter swims were held in municipal indoor pools in Britain and abroad, and she was supported by the community- singing of the thousands of people who attended her performances. It was the fees she earned from these endurance swims that enabled her to build up her charitable trust fund – income from the open water swims being sporadic, sometimes only covering expenses. .
Mercedes retired from swimming in 1933 and thereafter lived a reclusive life as a housewife and mother to her three children.