Cheat: Infinite Lives on World 4-5

To get infinite 1-UPs, you needn’t go any further than World 4-5. There, head to the first pipe spawning Bobombs. Clear a way to the lower (indestructible) floor, but leave the left and right edges in tact. Grab a Koopa Shell and toss it into the pitt, and then position yourself far away enough from the pipe so the Bobombs continue to respawn. If done correctly, the Koopa shell will bounce left and right endlessly, racking you up 99 lives in no time.
Josh
Cheat: World 8 (From World 5)

To get to World 8 from World 5, do the following. First, head to the house directly after the first castle on World 5. Once there, go up six flights of stairs. Next to the seventh flight of stairs, there will be three bricks. Jump on top of them and jump straight up to hit an invisible block. Then, get on top of that block, jump straight up again, and hit another block. This one will unleash a vine that you can climb up. Once atop the vine, there will be a door. Go through the door and get on the nearby colorful platform. The platform will bring you to another door. Go through that door and find the red flag with a skull on it (as opposed to a black flag). Jump on the flag so it comes down, thereby completing the stage. Then, go to the cannon it shows and you will be brought to World 8. (unverified)
Toady, Cole Sprouse
Unlockable: Play as Luigi

Beat the game and save it. Then, select the saved game and hold down both the L and R BUTTONS, then enter A. The green sporting super mario bro is unlocked.

I can confirm that you do not need to clear the game to play as Luigi. Just hold L + R and press A on the file you want to play, and you will be Luigi, regardless if you finish the game or not.
SymbolicGamer
Unlockable: Special Backgrounds

First you beat the game. Then load the saved file for that game. Go into the blue Toad house now located in World 1. In here you can purchase five different backgrounds for your lower screen. They all cost Star Coins. To get the fift background, you have to get all of the star coins in the through-out all the levels.

Toady claims to get the fifth background you must collect all star coins. This is wrong. You only need to clear a path to every mushroom hut. Finish the game and a path to a blue mushroom hut will appear in World 1. It contains the starting touchscreen theme and three additional background patterns that can be purchased for 20 star coins each. A fifth theme can be bought once you've cleared paths to the mushroom huts in every world.
Toady
Unlockable: Unlimited Toad House Items

Cleat all the levels and lettered bonus levels in the game and save it.
crispylatino101
Unlockable: World 4

Beat the boss in the last castle of World 2 as Mini Mario.
FantasyFiend
Unlockable: Final Background Pattern

You must complete the game and collect every star coin on every level. Buy it for 20 star coins.
MRSlle
Unlockable: Challenge Mode

Once you complete the game, go to the world map and enter Start, L R L R X X Y Y.
Aidan
Unlockable: World 7

To unlock world 7, do one of the following:

1. Defeat the World 5 Castle Boss as Mini Mario or...
2. Unlock the World 4 Cannon (you need to finish the W4 Haunted House with a RED FLAG)


Toady
Hint: Extra Toad Houses

At the end of the levels, jump onto the flagpole and when the last two digits of your countdown timer are the same. For Red Houses, jump on at 11, 22, or 33. For Green Houses, jump on at 44, 55, or 66. And for Orange Houses,
Hint: Getting Power-Ups

For getting more power-ups fast, go back to World 1 and play the stages with the red winged question mark blocks.
Hint: Warp to World 5

There's a warp pipe that'll instantly bring you to World 5 from the World 1-Tower. When in the hidden room with the second Star Coin, notice a small gap in the upper-right corner of the room that's never closed by the shifting blocks. You can jump into the gap to reach a hidden area with a 1-Up (smash the invisible blocks as soon as you enter the room). Shell Mario can go one further, breaking the blocks along the floor to reach the warp cannon.

Hint: Infinite 1UPs

At the pipe right at the end of World 1-4, kick a koopa shell and it'll keep sliding back and forth between the pipe to the right and the wall to the left. If you jump every time the shell comes your way, the shell will kill the enemies that constantly appear, eventually giving you you 1UPs.
everything aol
Hint: House Exploitation

First, each time you get to a boss stage, return to world 1. Play the first stage once through, getting all the power ups but the giant mushroom (you are forced to only be super mario at the end of the level if you gat it, without the fireballs). Instead, beat it and jump on the flag pole when the last two digits of the clock are either 66, 55, or 44. This way, a toad house with extra lives will appear. Repeat this until you have many, many lives to spare.

To cheese the bosses, play through one more time (without getting the big mushroom) and end when the last two digits of the clock say 99, 88, or 77. This will make a toad house with the giant mushroom powerup appear. It will replace whatever else you already have. Finally, go to the stage with your newly equipped giant mushroom, fireballs, and lives. Play through it, but don't use your giant mushroom yet.

When you get to whatever boss you are playing, immediately activate the giant mushroom. Now you can defeat the boss just by touching him! I don't know if the big mushroom will kill all bosses, but for most bosses, this works.
Hint: Free Mushroom Houses

Instead of immediately jumping onto the flag pole at the end of a stage, take your time and watch the clock. If you hit the flag when the last two digits of your timer are the same (example: 133, 211, or 099) you'll unlock a mushroom house on your World Map. If the last two digits are 1, 2, or 3, you'll unlock a red mushroom house. If the last two digitsl are 4, 5, or 6, you'll unlock a green mushroom house. If the last two digits are 7, 8, or 9, you'll unlock an orange mushroom house.

Hint: Warp Zone Cannons

In some worlds, there are cannons you can access (usually through taking secret exits). When you enter the cannon area, there will be a large cannon in the area, and a world number written below it. If you want to warp, simply drop into the cannon and off you go! Here is a list of the known warp zones thus far:

World 5 Warp #1: Take the secret exit from the World 1 Tower.

World 5 Warp #2: Take the secret exit from Stage 2-A.

World 6 Warp: Take the secret exit from the World 3 Ghost House.

World 7 Warp: Take the secret exit from the World 4 Ghost House.

World 8 Warp: Take the secret exit from the World 5 Ghost House.

Hint: Easily Defeat Bowser and Other Bosses

If you're having trouble defeating a boss, bring a Mega Mushroom into the stage. When you fight the boss, activate the Mega Mushroom and simply stomp all over the enemy to defeat him. This is especially useful at the end of the game where you have to fight two enemies at once.

Hint: Item Gauging

To get a particular item from the red mushroom houses then hop into one. When Toadsworth starts the slot go under it. Here's the tricky part: pause the game two items before the one you want and double tap A or B. For example, if you want a mini mushroom, pause the game at the flower and press A or B twice. Viola! You get the mini mushroom.
feezymang
Hint: Bonus Mushroom House

If you complete any stage when the last two digits of your countdown timer are the same (ex. 177, 233, 08 a bonus mushroom house will appear on the far left side of the world map. Stopping the timer with 11, 22, or 33 will give you a red mushroom house with a random item box. A timer with 44, 55, or 66 will give you a green mushroom house that rewards you with 1-Ups. Stop the timer at 77, 88, or 99 and you'll unlock an orange mushroom house that gives you a Mega Mushroom.
andrew_yang33
Hint: Infinite 1-Ups (World 2-2)

This is easiet when you are blue shell Mario. In World 2-2, go past the three turtles, then past the cluster of power-ups, to find four coins enclosed by a cage of blocks.

When you take those out, you create a small, walled up gap where if you throw a shell into it, it will bounce back and forth forever. You would already have been attacked by a creature in a cloud throwing spiked shells at you. Stand close to it without stepping on the shell, and let the cloud creature keep tossing the spikeys into the gap, where the shell bouncing back and forth will take them out.

Press down (crouch) to hide in your blue shell so you don't get hurt. Soon, every spikey that's thrown into the gap and taken out by the bouncing shell will give you one more life. You'll reach 99 lives in no time.

In principle this works wherever you can get a shell to bounce back and forth perpetually, and there's an endless supply of enemies dropping in to be taken out.

Hint: Easy Save

Above World 1-1 there is a house where you can change your background. Everytime you've changed your background, you can save. Note: You'll need 20 Starcoins before you try this.