Under a sink is the most reliable place in a house to find mould, and the reason is structural rather than hygienic. It is a dark enclosed box, it contains the two things that leak most often, a trap and a supply connection, and the cabinet floor is usually particle board, which is sawdust and glue and behaves like a sponge. A slow drip that nobody notices for a month is all it takes, and by then the question is usually about the cabinet rather than about the mould.
Where the water comes from
Four usual sources, in rough order of frequency: the trap or its slip joints, the supply connections and shut off valves, the sink rim seal where water runs down the outside of the bowl rather than into it, and condensation on a cold supply line in a humid room. A fifth, in kitchens, is the dishwasher connection. Each leaves a different pattern, and the pattern is worth reading: a stain directly under the trap, a stain at the back wall, a ring around the rim cut out, or general dampness with no defined source.
Mold under sink particle board: why the cabinet usually loses
Mold under sink particle board is a material failure as much as a growth problem. Particle board swells irreversibly, loses its bond, and holds moisture deep inside where nothing can be cleaned or dried. Once the cabinet floor has swelled at the edges or gone soft, the honest answer is replacement of the base rather than treatment. Solid timber and plywood carcasses do much better and can often be cleaned, abraded and dried. Wood mold under sink on a solid carcass is usually a surface problem on a sound substrate.
Yellow mold in bathroom cabinets, black mold in sink drains: the colours
Black mold under sink and black mold in sink drains, yellow mold in bathroom cabinets, red mold bathroom staining and the pink film people find in showers all describe the same underlying situation: a damp, dark, food rich surface. Pink and orange films are frequently bacterial rather than fungal. Growth inside a sink bowl or drain is usually biofilm on the drain rather than building mould. None of these change what has to happen, which is to stop the water, remove the growth, and replace whatever cannot be cleaned through.
When it is more than a cabinet
If the leak has run for long enough, it will be in the floor and the wall rather than only in the box. The signs are a soft or discoloured floor at the cabinet kick, staining at the base of the adjacent wall, a musty smell that persists after the cabinet is cleared, cupping in an adjacent wood floor, and a stain on the ceiling below on an upper storey. At that point it is a water loss with a mould consequence, it is worth a moisture meter rather than a sponge, and it may be worth a claim.
Questions people ask about mold under sink
Is black mold under kitchen sink cabinets dangerous?
Indoor growth should be removed and the moisture corrected whatever its colour, and sensitised or immunocompromised people react more. The colour matters less than how long it has been wet and how far the water has gone.
Can I save a particle board cabinet base?
Rarely, once it has swelled or gone soft. Particle board loses its bond permanently and holds moisture where it cannot be dried. Plywood and solid timber carcasses are frequently recoverable.
What is the pink or orange film in my bathroom?
Usually a bacterial biofilm rather than a fungus. It is a sign of persistent moisture on a surface, and the action is the same: remove it and reduce the dampness that feeds it.
When is it more than a cabinet problem?
When the floor at the kick is soft, the adjacent wall base is stained, the smell persists after the cabinet is cleared, or there is a stain on the ceiling below. That is a water loss, not a cleaning job.