It was based on a version of the Supermarine Baby, the first single-seat flying boat fighter aircraft to be designed and built in the United Kingdom, that first flew in February 1918.

Registered as G-EALP, the Sea Lion competed successfully against an Avro 539 to win a place as one of the British entrants.

On 10 September, the day of the race, thick fog obscured the route and caused the Sea Lion's pilot Basil Hobbs to make an unscheduled landing on the water.

The Sea Lion I was developed to participate as the British entry in the 1919 Schneider Trophy race, that last taken place in 1914, when Britain had been the victors.

[7] The hull of the N61, which was in the form originally designed by Linton Hope for the AD Flying Boat, may have been used for the Sea Lion I.

The Sea Lion took the opportunity to be fitted with a newly-arrived propeller, after the Avro was accidentally damaged before the start of the trials.

The undamaged planes set off, but the thick fog obscured the route of the race, and mid-flight the Sea Lion's pilot, Squadron leader Basil D. Hobbs, was forced to land to recalculate his bearings, which he had lost over Swanage Bay.

The damaged aircraft alighted near Bournemouth Pier for the compulsory first lap landing, but inverted with its tail stuck up in the air and sank, and Hobbs was forced to abandon the race.

Hobbs was picked up from the site of the wreck, which was towed to Boscombe Pier and returned to Supermarine's works at Woolston, Southampton.