Leiston FC are another of Diamonds opponents who have been around since 19th century. Founded in 1880 and like many at that time linked to a local industry in this case Engineering Works. Playing mostly in local Suffolk leagues they moved through Eastern Counties League and into the Ryman League in recent years before being allocated to Southern League Premier Central. A best finish at Step 3 was achieved in 2017/2018 season with a 5th place finish and a play-off tie which saw a close defeat to Dulwich Hamlet.
Leiston made FA Cup First Round Proper in 2008 as one of only three teams to have made that stage from the Extra Preliminary round. Fleetwood Town were the opponents who were held to a home draw in front of over 1,200 before defeat in the replay.