From the Super Mario Wiki
The Golden 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 when Mario collects it, he gets 50 coins (or 100 coins if he is Gold Mario) upon contact. It lacks the crown from its contemporary Mario Kart appearances and the colouring is reversed from all other occurrences, with the sports 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 manual, it is called Super Mushroom. In the European version of Mario Kart 7, it is called Golden Dash Mushroom.
Mario & Luigi series
In Mario & Luigi: Superstar Saga, it is used as a Mushroom and a Syrup in one item, restoring both HP and BP to max for a single brother. Though there are only five Golden Mushrooms in-game, a glitch that happens with Fawful or Popple and Birdo can net the player with an infinite amount of them. The player must use Swing Bros. and get the mushroom then get a game over and return to the place. The player keeps the golden mushroom, and can repeatedly do that as long as they wish.
In Mario & Luigi: Partners in Time, the Mushroom Drop is also golden and is the game's Nut. They heal the Mario Bros. and the Baby Mario Bros. 15, 30, and 60 HP for the normal, Super, and Ultra versions, respectively. Although they aren't called golden mushrooms, their appearance is very similar to that of most of the golden mushrooms.
Mario Party series
In Mario Party 2, 3, 5, 6 and Mario Party 7, it allows players to roll three Dice Blocks at one time on their turn, (or in Mario Party 5, on their next turn if they landed on a space containing it), instead of just the standard one Dice Block. In Mario Party 4, it is replaced by Super Mega Mushroom, which serves a similar purpose. In Mario Parties 2-7, the user gets 20 coins for a triple number and a 50 coin bonus for triple 7s. In the 8-player mode in Mario Party 7, if two players roll the same numbers on all six dice blocks, they will get 300 coins. In Mario Party: Island Tour, known as Golden Dash Mushroom, it allows the player to add five spaces to the Dice Block roll.
Mario Party 3 features the Golden Mushroom as Mario's favorite item.
Captain Toad: Treasure Tracker
Golden Mushrooms, called to Gold Mushrooms, will appear as rare collectables in some courses in Captain Toad: Treasure Tracker. They will be worth a total of 50 coins similar to their New Super Mario Bros. 2 variants. These mushrooms will appear with out the crown.
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