Direct answer: If mice are showing up around the pipe under the sink, the usual problem is an open wall or cabinet penetration around the drain or supply lines, not the pipe itself failing. Start by finding whether you have a simple gap, a loose escutcheon, or actual chewed pipe or wall material.
Most likely: The most likely cause is a rough oversized hole where the pipe passes through the cabinet or wall, often hidden behind stored items or a loose trim ring.
Under-sink mouse entry is usually a hole-sealing job first and a plumbing repair second. Reality check: mice only need a surprisingly small opening. Common wrong move: packing the area with soft filler alone and calling it done. You want to expose the opening, check for active leaks or chew damage, then seal the gap with something mice won’t shred while keeping the pipe free to move and serviceable.
Don’t start with: Don’t start by foaming every opening shut or buying plumbing parts before you know whether the pipe, the wall opening, or the drain connection is the real issue.
There’s an obvious gap where the pipe passes through the cabinet back or wall, but the pipe itself looks normal.
Start here: Start with a close visual check and measure the opening before sealing anything.
The cabinet wall, trim ring, or nearby plastic piping has tooth marks, shavings, or rough edges.
Start here: Check carefully for actual pipe damage and leaks before treating it as just a wall-gap problem.
You notice mouse activity along with a musty, urine, or sewer-like odor near the pipe opening.
Start here: Separate pest odor from a drain leak or open drain gap before sealing the area.
The opening may be partly hidden, but activity seems to center around the plumbing penetration.
Start here: Empty the cabinet fully and inspect every pipe pass-through, especially behind the drain trap and shutoff valves.
This is the most common setup: the installer cut a rough hole larger than needed, and mice use that edge as a path from the wall cavity into the cabinet.
Quick check: Remove stored items and look behind the drain trap and supply tubes for a visible crescent-shaped gap or open annular space around the pipe.
A trim ring can slide away and expose a hole that was always there, making it look like the pipe suddenly became the problem.
Quick check: See whether the decorative ring around the supply line or drain stub-out is loose, cracked, or missing and whether the wall opening behind it is much larger than the pipe.
Mice will gnaw some plastics, especially where a pipe passes through a rough opening or rubs against wood. That can turn an entry problem into a leak problem.
Quick check: Run a dry paper towel around exposed plastic piping and flexible supply lines and look for wetness, scoring, pinholes, or flattened chew marks.
Sometimes the pipe opening under the sink is only the last visible spot. The real entry is a bigger gap lower in the cabinet, at the floor, or behind the back panel.
Quick check: Use a flashlight to inspect the cabinet floor corners, toe-kick area, and the lowest pipe penetrations for larger hidden openings or droppings.
You need to see the first opening clearly before you decide whether this is a simple sealing job or damaged plumbing.
Next move: You can now point to the exact opening or damaged area instead of guessing from where the mice were seen. If the cabinet is finished tight but you still hear activity, the opening may be behind the back panel, in the wall cavity, or at the floor line nearby.
What to conclude: Most homeowners find the problem once the cabinet is fully emptied and the drain trap area is lit properly.
Sealing a hole is fine. Sealing over a chewed drain or supply line just hides a leak until the cabinet rots.
Next move: If the pipes stay dry and only the surrounding opening is exposed, you can move on to sealing the entry point. If you find chew damage or leaking on the piping itself, fix the damaged plumbing first and seal the gap after the pipe is sound.
What to conclude: A dry intact pipe points to an entry-gap problem. Wetness or chew damage means the plumbing needs repair before closure.
A loose escutcheon is a common lookalike. Sometimes the fix is as simple as exposing the real hole and sealing it properly, then sliding the trim back into place.
Next move: You’ve confirmed the trim was hiding the real entry point, and the pipe itself is not the repair. If the hole is large, irregular, or extends into a bigger cavity, you’ll need a more solid exclusion repair than a trim ring can provide.
Once the pipe is confirmed dry and intact, the goal is to block the path without damaging the pipe or making future service impossible.
Next move: The opening is physically blocked, the pipe remains accessible, and mice no longer have an easy path into the cabinet. If the gap is too large, the wall material is crumbling, or the path continues into a bigger hidden void, bring in pest exclusion or a plumber/carpenter to rebuild the opening correctly.
If mice used this opening once, there is often another nearby route that fed the wall cavity or cabinet in the first place.
A good result: No new droppings, scratching, or sightings show up, and the under-sink area stays dry and closed off.
If not: Persistent activity means the sink opening was only part of the route, or there is another unsealed penetration nearby.
What to conclude: You’ve either solved the cabinet entry point or confirmed the mice are coming from a larger building-envelope gap that needs broader exclusion work.
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Yes. If the wall or cabinet opening is oversized, mice can use that gap even when it looks too small at first glance. The visible pipe is often just the route marker, not the failed part.
Not as the only fix where mice are active. Soft filler alone is easy for rodents to chew. First confirm the pipe is dry and intact, then use a rodent-resistant physical block and treat trim or sealant as the finish layer.
Dry the pipe, run water, and watch closely. Fresh drips, wet paper towel marks, pinholes, or obvious gnaw marks on plastic piping mean you have a plumbing repair to handle before sealing the opening.
The loose escutcheon itself is not the entry point. It usually just exposes the larger wall hole behind it. If the opening behind the trim is open, mice can use it.
Then the under-sink gap was probably only one part of the route. Check nearby floor penetrations, toe-kicks, adjacent cabinets, basement or crawlspace access points, and exterior entry gaps. Ongoing activity usually means a larger exclusion job is still open.