Jarvis' rising weight made him unsuitable as a flat race jockey and after briefly competing under National Hunt rules he retired from riding in his early twenties.

[1] Jarvis set himself up at Warren House stable, Newmarket as a private trainer for A. E. Barton in 1914, but after two years the yard was closed because of the First World War.

Jarvis had deliberately misled Rosebery about Ellangowan's condition in 1922: he wanted to give the colt time to develop and discouraged the owner from racing him by claiming that the horse had a recurrent coughing problem.

[2] During the war, Jarvis' best horses were the filly Ribbon who finished second in three classic races and the colt Ocean Swell who won the substitute Derby at Newmarket in 1944.

Immediately after the war he trained Royal Charger, owned by his namesake Sir John Jarvis, who won the Ayr Gold Cup in 1946 before becoming a highly successful breeding stallion in the United States.

[2] There were no classic winners among them, with his best horses being the handicapper Fastnet Rock and the two-year-old filly Primavera, who won the Queen Mary Stakes at Royal Ascot.

In 1955 Jarvis threatened to retire after the Jockey Club attempted to introduce new rules making trainers liable for any delays caused by adjustments to a horse's girth strap before the start of a race.

At the time, Park Lodge stable housed a two-year-old filly named Sleeping Partner who went on to win the following year's Epsom Oaks, the only classic to have eluded Jarvis during his training career.