My lawn is boggy perhaps due to being on an incline and getting runoff from garden at back. However the problem now is that when I mow the lawn large patches of black wet spawnlike fungus/algae(?) have seeped up. It looks like black frogspawn. This is the garden of a new house so I imagine that the soil is just whatever the builder threw down and sowed lawn seed over.
If you have no drainage outlet, you might consider a soak-pit to take water off the surface.