The Martian Leader is as equally insane and destructive as every other Martian in the movie. Not only is he a mastermind, he is also a rigid political leader, albeit a little stereotypical: he does nothing more than staying on his ship and giving orders, smacking his minions when they fail, and creating strategies and schemes to destroy Earth's government. Although he is a very influential and authoritative figure in his army, he is only seen interacting with one of his henchman, the Martian Ambassador, and has never interacted with any human character in the movie with the exception of reporter Natalie Lake (whose head was being used in his experiments), General Decker (whom he later shrunk and squashed with his foot after he shot at him), and President Dale (whom he tricked by giving him a robotic arm which burst a hole through his chest).
His first appearance is after the Nevada Desert attack, when President Dale and Professor Kessler try to send the Martians a peace offering, which is translated to their language. However, once the Ambassador shows the message to the Leader, they burst out laughing and pretend to accept it. Their plan succeeds when the US Congress is destroyed and Professor Kessler gets kidnapped.
He is later seen watching the Martian Girl's attempt to assassinate the president, but goes berserk once "she" is gunned down by the president's secret service, ordering a massive attack on Washington D.C. After the First Lady is killed and President Dale's daughter, Taffy, goes missing, they activate the USA's nuclear arsenal in hopes of destroying the Leader's saucer, but fails again. When the Martian Leader corners President Dale, having killed off most of his defense force, the president decides to stand firm and makes a heartfelt speech (this is a particularly focal point in the movie, as up until that point the president had always read from a prepared statement). The Martian Leader looks genuinely touched by this, as he sheds a tear and extends his hand in friendship. When President Dale shakes it, the hand detaches to reveal that It was a robot hand. The hand then stabs the president with its metal tail and sticks out of his corpse, a flag pops out of its tail declaring the planet in the name of Mars.
The Leader is seen again in the ending, when outcast Richie Norris and his grandmother broadcast high-frequency yodel music (Slim Whitman's Indian Love Call) that is sent worldwide, blowing up the Martian's heads. The Leader is the final Martian to die, as his head finally explodes, his saucer crashes into the water, also killing Donald Kessler and Natalie Lake.
General Decker - Made tiny with a shrink ray and then squashed under his foot.
President Dale - Pretended to be remorseful and then detached a robotic hand to stab the President from the back to the chest.
It is unknown if he is the leader of all of the Martians on Mars or simply the leader of the group of Martians invading Earth.
The Martian Leader's only goal in-mind is to kill President Dale for some reason. Once he accomplishes that, he is not seen much in the movie afterwards other than his death scene.