From the Super Mario Wiki
The Mega Mushroom is a rare variety of mushroom found in the Mushroom Kingdom. It enlarges the user, allowing them to walk over and destroy everything in their path, including the largest and most powerful enemies. Mega Mushrooms made their debut in Mario Party 4, where they were depicted as normally proportioned mushrooms with green caps; however, their current appearance is quite different and unique among the Mushrooms of the Mario franchise.
Introduced in New Super Mario Bros., the current Mega Mushrooms are now much bigger, with bulbous, red-spotted orange-yellow caps, and while all other mushrooms slide along the ground, the Mega Mushrooms hop. Originally, they would actively flee from Mario, and if the player jumped over one, it would even change directions, although this trait was omitted from the Mega Mushroom's New Super Mario Bros. 2 appearance. Its opposite counterpart is the Mini Mushroom, which miniaturizes the player.
Mario Party series
Mario Party 4
Mega Mushrooms first appear in Mario Party 4, with a light green cap (resembling a 1-Up Mushroom). It can be obtained by landing on a Mushroom Space, purchasing it from an item shop, getting it from an Item Bag, or winning a mini-game on the board. They let the player roll two Dice Blocks at once, making the player grow to an overwhelmingly gigantic size and flatten other players as they stomp across the board, stealing ten coins from them. However, board events will be bypassed (except for events involving Bowser at Bowser's Gnarly Party). If the player gets the same number on both Dice Blocks the player gets 10 coins (30 coins for two "7's"). Additionally, a Super Mega Mushroom is available, colored entirely translucent blue. It does the same thing but with three Dice Blocks.
Mario Party 7
A prop resembling a Mega Mushroom can be seen on the stage in the minigame Clock Watchers.
Mario Party 10
Mega Mushrooms make a comeback in the series for Mario Party 10. Mega Mushrooms only appear in the amiibo Party mode and act similar to how they did in Mario Party 4, where it'll make the user grow and cause the others players to lose some coins upon passing them.
Super Mario series
Super Mario 64 DS
While the Mega Mushroom does not make an appearance in Super Mario 64 DS, the Super Mushroom has the same effect as the Mega Mushrooms of later games, as it temporarily makes the characters giant, greatly reduces their jumping abilities, and allows them to destroy nearly anything in their path. Destroying enough objects will cause numbers to appear above the character's head, and eventually, every object destroyed will earn the player an extra life.
New Super Mario Bros.
Mega Mushrooms later reappear in New Super Mario Bros., where they again make Mario grow to a giant size. In this colossal form, Mario can stomp across the level and crush anything in his path - enemies, blocks, Warp Pipes, and even the Flagpole at the end of the level. When items are crushed by Mario, the bar at the top (referred to as the Mega Meter in the instruction manual) starts to fill up with yellow. As the yellow line reaches the end of a segment, that segment turns green. After Mario's size reduces again, a 1-Up Mushroom appears for each green segment filled, with a maximum of five 1-Ups. These mushrooms are rarely found, and almost always come from Toad Houses or Red Flying Blocks. Besides the Toad Houses, they only appear in four levels: World 1-1, World 1-2, World 4-4 and World 7-5. If Mario or Luigi ground pounds a block containing a Mega Mushroom, it still appears from above. The player is not completely invincible in this form, however; if the player is to jump and land on a hole covered with bricks, the player falls through, thus, losing a life, if the hole is big enough. In other cases, Mario, or Luigi, lose a life if they fall in lava or poison water. Additionally, if Mario or Luigi are to do a Ground Pound after eating a Mega Mushroom, a group of Goombas or Coins (or both) falls from the sky.
In the E3 2005 trailer build and at least one of the 2005 demo builds of New Super Mario Bros., the Mega Mushroom had the same color as the Super Mushroom, but it was changed to yellow and red in the final version. This was possibly meant to resemble the original colors of a Super Mushroom from Super Mario Bros..
New Super Mario Bros. 2
The Mega Mushroom returns in New Super Mario Bros. 2 after their removal from New Super Mario Bros. Wii. They retain the same function as in the first New Super Mario Bros. Like in New Super Mario Bros., they are very rare, only appearing by launching some fireballs in a black pipe from World 1-2, World 5-6, or World Mushroom-3; by doing so, a Mega Mushroom will appear (though one in a ? Block does appear in the third course of the Gold Rush Pack). In addition, though Mario cannot gain 1-Ups for the destruction he causes, he can get three 1-Up Mushrooms by destroying the Goal Pole; this can only be done in World 5-6. Unlike in New Super Mario Bros., Mega Mushrooms aren't found in Toad Houses and cannot be put in reserve.
Super Mario 3D World
The Mega Mushroom makes a return in Super Mario 3D World, making it the item's first appearance in a 3D Mario title. It is the same as in the handheld New Super Mario Bros. games. However, if someone ground pounds, they can now create a small wave that breaks all nearby blocks and enemies. As in the other games, Mega characters can destroy Warp Pipes.
Just as the Mega Mushroom is about to wear off, unlike in New Super Mario Bros. or New Super Mario Bros. 2 where the character would emit a green light, the music goes a little over-pitched.
Mario Kart Wii
In Mario Kart Wii, the Mega Mushroom is an item obtainable from the Item Box. When players use this item, they grow bigger and their voice gets much deeper. This item offers the players the ability to squash other players for a certain amount of time, reducing their speed and making them lose their items. While this item is active, the players receive a speed boost and cannot slow down when going off-road, and they are invincible to all dangerous things, except for the Star, Bullet Bill and Blooper, giving them the power to destroy any items or any course obstacles, except those that are big and metallic (such as Chain Chomps). If players get hit by Lightning while they are big, they shrink back to normal size rather than becoming tiny. The effect also ends prematurely if players fall off the course. The Mega Mushroom's theme music from New Super Mario Bros. is played for the duration of the item's effect.
Trading Card Info
"The Mega Mushroom is a new item this time around in the Mario Kart series. When activated, you and your vehicle will become super-sized for about 10 seconds and will be immune to all attacks from other racers...except those wielding Star power, Bullet Bill or Lightning. While under the influence of Mega Mushroom power you can run over your rivals and flatten them like a pancake!"
Super Smash Bros. Brawl
The Mega Mushroom appears as a sticker in Super Smash Bros. Brawl. It raises food effect by 4.
Mario & Luigi series
Mario & Luigi: Bowser's Inside Story
In Mario & Luigi: Bowser's Inside Story, during his Vacuum command tutorial, Broque Monsieur sometimes attacks by using a Mega Mushroom to make himself giant. Bowser needs to punch his body to avoid being rammed. This will knock Broque Monseiur's (still giant) head off, so Bowser can punch it to earn extra coins until his body picks it up again.
Mario & Luigi: Paper Jam
Mario & Sonic at the Olympic Winter Games
In Mario & Sonic at the Olympic Winter Games, the Mega Mushroom appears in the Dream Event, Curling Bowling, and Balloon Blast. The actual item does not appear, but there is an Item Box that it could be obtained from. The Mega Mushroom makes the character 2x his or her original size.
Mario Tennis: Ultra Smash
Mega Mushrooms appear once again in Mario Tennis: Ultra Smash, where they make characters big, allowing them to hit the ball more easily. Instead of the original music from New Super Mario Bros., a rock cover of the original tune plays when using the Mega Mushroom. The effect lasts for 30 seconds, but can end prematurely if the player gets hit by the ball. Three seconds before the item wears off, the character flashes white before shrinking back to normal size.
Sprites and Models
Names in other languages