Jarvis made his debut as a wing-half for West Ham United on 2nd April 1904. The team at the time included Tommy Allison, Matt Kingsley, James Bigden, Billy Bridgeman, Fred Griffiths, Charlie Satterthwaite, George Hilsdon and Herbert Lyon.
In 1905 Syd King signed George Kitchen from Everton. An outstanding goalkeeper he was considered to be one of the best in England. He joined a defence that included Jarvis, Tommy Allison, Frank Piercy and David Gardner.
At the start of the 1906-07 season was marred by a game against Millwall. The East Ham Echo reported: "From the very first kick it was seen that there was likely to be some trouble. All attempts at football were ignored." According to the report, Len Jarvis smashed Alf Dean against a metal advertising board. Dean was so badly injured he was unable to continue. The Stratford Express commented: "This caused a deal of feeling between the sides."
Len Jarvis was cautioned by the referee for his tackle on Alf Dean but a subsequent investigation by the Football Association resulted in the player being suspended for 14 days. The supporters were also criticised for the fighting that took place in the stands. West Ham was also forced to post warning notices about future behaviour at home matches.
After scoring 5 goals in 133 games Jarvis was transferred to Bury in 1909. Over the next three years he played 55 for his new club.