From the Super Mario Wiki
The Golden Mushroom (sometimes referred to as a Gold Mushroom) is an item that appears in several Mario games. It is a rare Mushroom, sometimes depicted with a crown on its head, and it always gives the player a positive effect when used. Golden Mushrooms sometimes vary by color and luster between games, but they are always a strong item.
Super Mario series
Super Mario World
New Super Mario Bros. 2
Gold Mushrooms appear in New Super Mario Bros. 2 in the Coin Rush mode and replace 1-Up Mushrooms, and upon contact, Mario gets 50 coins (or 100 coins if he is Gold Mario). It lacks the crown from its contemporary Mario Kart appearances and the coloring is reversed from all other occurrences, with the spots being darker than the background.
Mario Kart series
The Golden Mushroom appears in Mario Kart 64, Mario Kart: Double Dash!!, Mario Kart DS, Mario Kart Wii, Mario Kart 7, and Mario Kart 8. Since Mario Kart 64, it gives the player an endless amount of speed boosts for a limited time. Additionally, in Mario Kart: Double Dash!!, it is Toad and Toadette's special item. The Golden Mushroom's initial appearance is a glowing gold Mushroom, but since Mario Kart: Double Dash!!, it has been given a gold crown with red jewels as well. This item was originally going to be in Mario Kart: Super Circuit, but never made it to the final game.
In the Mario Kart 64's Operation Card, it is called Super Mushroom. In the European version of Mario Kart 7, it is called Golden Dash Mushroom.
Mario & Luigi seriesIn Mario & Luigi: Superstar Saga, it fully restores both Heart Points and Bros. Points for a single brother. There are only five Golden Mushrooms in-game, though it can also be stolen using Swing Bros. from Fawful, Popple, and Birdo.
Mario Party series
In Mario Party 2, 3, 5, 6 and Mario Party 7, the Golden Mushroom (or, in the latter three games, the Super Mushroom Orb or Super 'Shroom Orb) is an item that allows players to roll three Dice Blocks on their turn, (or in Mario Party 5, on their next turn if they land on a space containing it). In Mario Party 4, it is replaced by Super Mega Mushroom, which serves a similar purpose. Rolling three of the same number earns the player 20 coins, and rolling three sevens earns the player a 50 coin bonus. In the 8-player mode in Mario Party 7, if a team rolls the same number on all six dice blocks, they will get 300 coins.
In Mario Party: Island Tour and Mario Party: Star Rush, an item known as Golden Dash Mushroom appears, which allows the player to add five spaces to their roll. In Mario Party: Star Rush, players have to sort between Mushrooms and Golden Mushrooms in the Coin Chaos minigame, Conveyor Meltdown.
In Mario Party 3, the Golden Mushroom is stated to be Mario's favorite item.
Captain Toad: Treasure Tracker
Golden Mushrooms, referred to as Gold Mushrooms, appear as rare collectables in some courses in Captain Toad: Treasure Tracker. Collecting one gives the player 50 coins similar to their New Super Mario Bros. 2 appearance. These mushrooms do not have a crown and are usually a secondary objective in the courses it appears in. Some Gold Mushrooms can only be found through invisible Pluck Patches, which can be seen once Captain Toad approaches it.
Puzzle & Dragons: Super Mario Bros. Edition
Golden Mushrooms are rare items in the puzzle and RPG hybrid called Puzzle & Dragons: Super Mario Bros. Edition. It is obtained from ? Blocks or dropped after defeating some enemies with a 10+ combo, such as 2-Goomba Towers and Paragoombas. There is an even rarer variation called Grand Gold Mushroom.
Profiles and statistics
Mario Kart 7 Game Manual
Gives you a speed boost whenever you press L/X, but only for a short time.
Mario Kart Wii trading card description
Golden Mushrooms generally show up in the Item Windows of racers near the back of the pack. It's an awesome tool for those unfortunate enough to be in that position as they can use it to catch up to those in front. Unlike other Mushrooms, however, you can tap the Item Button as many times as you'd like to maximize your boost. Be sure to cut corners where possible with this item.
Names in other languages