Enter your seed. Every woodland mansion appears on your real overworld terrain and the panel lists the nearest ones to the center of the map, up to 12,000 blocks out, with distance, direction and Nether coordinates for the trip.
Mansions are far apart, so zoom out and use the list. The Nether route (divide by 8) makes the trip much shorter.
The terrain is the same for both editions: since 1.18 one seed gives the same overworld on Java and Bedrock.
Mansions land in different spots on each edition, and the map shows the right ones for whichever you pick. Bedrock is also pickier: the whole area around a mansion has to be dark forest, so some spots that work on Java are empty on Bedrock.
Java positions are built on cubiomes, a re-implementation of Minecraft's world generator, and I checked them against worlds made by real Java and Bedrock servers. If you find one that's off, tell me in the comments with your seed and coordinates and I'll look into it.
Reference: Woodland Mansion Finder
Woodland mansions are the rarest regular structure in the overworld: enormous dark oak houses full of illagers, and the only place evokers spawn, which makes them the source of totems of undying.
Totems of undying
Evokers drop a totem of undying every time they die, and mansions are the only place evokers generate (besides raids). A mansion is usually good for two or three totems.
Getting there
Mansions are typically thousands of blocks away. Divide the coordinates by 8 and travel in the Nether, or bring a boat and follow rivers and coasts. The Nether coordinates are in the list.
How they are placed
Mansions use an 80 by 80 chunk grid with a triangular spread, and generate only where the biome is a dark forest. That combination makes them rare.
How do I find a woodland mansion?
Enter your seed and every woodland mansion appears on the map; the nearest ones are listed with distance, direction and exact coordinates. Mansions are rare and usually thousands of blocks from spawn, in dark forests.
Why are mansions so far away?
Mansions use a huge placement grid of 80 by 80 chunks (1,280 blocks) and need a dark forest, so the nearest one is often 5,000 to 10,000 blocks out. The list searches up to 12,000 blocks.
What is in a woodland mansion?
Evokers, which drop totems of undying, vindicators, allays waiting in cells, the vex armor trim, and chests with enchanted gear. Some rooms have secret loot.
Can I find a mansion without the seed?
Cartographer villagers sell woodland explorer maps that lead to the nearest mansion. With the seed, this finder shows it directly.
Does this work on Bedrock?
Yes. Bedrock uses the same 80 by 80 chunk grid but its own random positions and a stricter check: the whole area within 32 blocks has to be dark forest (or the caves under it). Pick Bedrock and the map shows those positions.