Robert Joseph MacCready

 is the mayor of Little Lamplight who came into office in 2274. A profane-mannered child with a no-nonsense attitude, he is armed with an assault rifle and threatens to "blow your fuckin' head off" when the Lone Wanderer and MacCready first meet. Despite his threat, he won't attack you when provoked. He'll simply run away shouting, "Every man for himself!" He is often referred to as RJ by other residents of Little Lamplight such as Princess and Eclair.

MacCready came to power shortly after the five-minute rule of Princess. According to Sammy, when Princess demanded the title of Mayor to be changed to Princess, MacCready punched her in the nose and said that the children of Lamplight needed a leader, not a princess. He was then elected to Mayor.

Lucy directs you to talk to MacCready if you want to trade for some Cave Fungus, which is a radiation medicine and food. MacCready will make you a deal to trade it for Buffout or Strange Meat if you have the Child At Heart perk, a high enough Barter skill (at least 50 to trade in 3 Buffout or 3 Strange Meat for 1 Cave Fungus, at least 70 to trade in 2 Buffout or 2 Strange Meat for 1 Cave Fungus), or you can intimidate him into giving you his private stash of 10 fungi by passing a speech challenge. (You will lose karma for this option, and he states that you shouldn't expect any good will in town after that, and he no longer discusses trading for more and none of the other occupants will trade with you.)

MacCready cannot be pickpocketed. He can't be killed because he is a child, and you can't kill children in Fallout 3 (other than Harden Simms and Maggie Creel in the Megaton explosion).

Quotes
Possibly the funniest line in the entire game.

(When you offer to make a deal to sell strange meat saying 'I've heard about your fungus')

"Yeah, I'm sure you've heard all about the 'fungus in my cavern.'"

Bugs
Mayor MacCready is prone to disappearing when you need him to continue a questline. Others have reported that he tends to wander into obscure and dark corners. He may appear near the front gate if you go outside and fast travel back to town. It may take a great deal of time to find him again (if you can at all) without resorting to the command console on the PC. The relevant PC console command is player.moveto 0000313B

While this may or may not be a "bug", it is advised that you get him to open the back gate during your first conversation with him, so seeking him out a second time is not required.

There have been some reports that sometimes MacCready will not give you the option to open the gate, even if you are at that point and have followed the advice outlined in this wiki. One way to make sure that this does not befall you is to click all of the grayed out speech options when speaking to MacCready. The dialog option to open the gate comes after a line that is often grayed/dimmed.

Related quests
Do you have to speak to the Kids that you rescued from Paradise Falls before talking to the Mayor RJ, in order for the backdoor option for Vault 87 to appear? I rescued the kids, and then went to little lamplight. I then was allowed into little lamplight, and RJ recognized that I was the kid hero for saving the kids, but no back door option appeared? If you need to get the backdoor open to enter Vault 87 in order to get the GECK so that you can complete the main quest in the game, I would say that this "bug" if it is one is very serious?
 * Picking up the Trail
 * Rescue from Paradise

Appearances
Mayor MacCready appears in Fallout 3.

Behind the scenes
"RJ MacCready" is also the name of the main character/hero, played by Kurt Russell, in John Carpenter's "The Thing." In the film, he also takes leadership of his group being there is no other option, and (much like his confrontation with Princess) personally punched out someone causing a problem

His personality resembles another character called MacReady who is also a mayor in the horror movie "Slither".

MacCready's physical appearance is that of the kid at the end of Mad Max Beyond Thunderdome. The kid is the son of The Gyro Captain from Road Warrior. Father, son, and the kids from the airplane crash site all fly off in his odd looking airplane. The quote "I'll blow your fucking head off" is exactly what the kid tells Mad Max and friends when he is "robbing" them.