Special Agent Mike Renko was a Field Agent assigned to the NCIS Office of Special Projects.
During an operation in Episode:Legend (Part 1) and Episode:Legend (Part 2) which acted as the series pilot for NCIS: Los Angeles and marked the debut appearances of all the characters, Renko went undercover for OSP and acted as "Mattie Rae", a customer who wanted to buy weapons. He revealed his true identity as an agent after a gunfight that left an arms dealer and his two guards dead while meeting Leroy Jethro Gibbs, the head of the Major Case Response Team at the Navy Yard in Washington D.C.
Later after G. Callen's meeting with Rivkin, Macy tasked Renko with tailing Rivkin with Renko agreeing to do it. A while later, Gibbs joined Renko after tailing Rivkin into a hotel and watching him have coffee in the gazebo where they discovered the body of another member of the terroist cell that Rivkin had killed before fleeing the scene.
During an operation, armed with rifles, Renko, Sam Hanna, and Kensi Blye stormed the shop where the target was located. The operation was successful, resulting in the capture of the leader of the terrorist cell and once the cell was dissolved, Gibbs and Special Agent Timothy McGee returned to D.C.
In the Season 1 episode, Episode:Ambush, Renko returned to OSP, having gone on an investigation for two months on a dangerous militia group called "Enhancing Law Enforcement", also known as "ELE", which fought illegal immigration ruthlessly. Rumor had it that the ELE were going to steal the "Dragon" missiles.
After his informer, Edward Mackaye was killed by Marine Corporal Scott Reilly, Renko joined the co-operation and during the recon on Riley's vehicle, Kensi was attacked by Riley who intended to kill her by garroting her. She managed to fight back and got the upper hand in the battle before Riley escaped. Renko, who had just finished checking Riley's mail box, showed up and arrested Riley with Kensi.
Later, Renko, Kensi, and Sam tracked down the camp of ELE. After being captured by the militia, Sam contacted with Kensi via Morse code. Sam told them to use "Maskelyne" and after learning what "Maskelyne" stands for, Renko and Kensi opened fire from several locations, making the militia believe that they had been surrounded by multiple units.
In Episode:The Bank Job, Renko made an arrangement for Callen and Sam to meet Detective Matt Bernhart. The four later worked together on a fake bank robbing.
In Episode:Burned, when Callen was absent, Renko was called to rejoin the team due to the lack of personnel and possible security concerns. He joined the operation to stop Ruman Marinov and his men from killing a German CEO. Renkoe was shot during the mission but survived because of the bullet proof vest he was wearing at the time. Later, he joined the operation to catch Eugene Keelson.
After being absent for most of the three seasons, Renko returned in the Season 3 finale, "Sans Voir" where it was revealed that he was working with Assistant Director Owen Granger on a case.
He was later brought to hospital after being in the jaw by a rogue suspect and tragically suffered a fatal cardiac arrest, resulting in Renko dying which left OSP devastated.