Launch from Cape Canaveral (KSC); landing at Cape Canaveral (KSC); ISS-ULF-6 ELC-3 / AMS.

This flight delivered the Alpha Magnetic Spectrometer and an ExPRESS Logistics Carrier to the International Space Station.

Work to replace a powerful magnet in a physics experiment named AMS bound for the International Space Station has forced NASA to move a summer flight (target launch date was July 29, 2010) by the shuttle Endeavour to mid November.

NASA announced on January 13, 2011 that astronaut Frederick Sturckow will serve as a backup commander for the STS-134 space shuttle mission to facilitate continued training for the crew and support teams during STS-134 Commander Mark Kellys absence. Mark Kelly's wife, Congresswoman Gabrielle Giffords, was critically wounded in a shooting on January 08, 2011 in Tucson, Arizona.

On April 04, 2011 the new target launch date was set for April 29, 2011. The delay removes a scheduling conflict with a Russian Progress supply vehicle scheduled to launch April 27, 2011 and arrive at the station April 29, 2011.

Following a two day solo flight Endeavour docked to the ISS on May 18, 2011. Joint operations with the ISS expedition 27 were performed.
The same day Express Logistics Carrier-3 (ELC-3), was handed off from shuttle Endeavour’s robotic arm to the International Space Station arm and attached to the left side of the station’s truss structure. ELC-3 holds spare hardware for future station use, including an ammonia tank, a high pressure gas tank, a cargo transport container, two S-band antenna assemblies and a spare arm for DEXTRE, the Special Purpose Dexterous Manipulator.

On May 19, 2011 the Alpha Magnetic Spectrometer-2 (AMS) was installed successfully on the outside of the International Space Station's right side. Mission Specialists Andrew Feustel and Roberto Vittori used the space shuttle’s robotic arm to extract it from Endeavour's payload bay. They handed it off to the space station’s Canadarm2, and Pilot Gregory Johnson and Mission Specialist Gregory Chamitoff then used the robotic arm to install AMS on the starboard side of the station’s truss.

The first EVA by Andrew Feustel and Gregory Chamitoff occured on May 20, 2011 (6h 19m). They retrieved two experiments and install a new package of experiments on ELC-2, which is already on the station. Next they installed jumpers between segments on the left-side truss, or backbone of the station, for ammonia refills; vent nitrogen from an ammonia servicer; and installed an external wireless communication antenna on the Destiny laboratory that will provide wireless communication to the Express Logistics Carriers mounted on the station's truss.

The second EVA was performed by Andrew Feustel and Michael Fincke on May 22, 2011 (8h 07m). The astronauts refilled radiators with ammonia. They also completed venting the early ammonia system, lubricated a left-side solar joint and parts of Dextre, a two-armed space station robot capable of handling delicate assembly tasks currently performed by spacewalkers.

The third EVA was again performed by Andrew Feustel and Michael Fincke on May 25, 2011 (6h 54m) to install a grapple, or handle for the robotic arm on the station to grab, on the Zarya module to support robotic operations based from the Russian segment. They also installed additional cables to provide backup power to the Russian portion of the space station.

The fourth and final EVA by Michael Fincke and Gregory Chamitoff occured on May 27, 2011 (7h 24m). Michael Fincke and Gregory Chamitoff stowed the shuttle's 50-foot boom on the right-side truss on a permanent stowage fixture, retrieved the grapple from the station's left-side truss and used it as a replacement for the grapple currently on the boom. They then released restraints from one of the arms on Dextre and replaced thermal insulation on one of the spare gas tanks for the Quest airlock.

The STS-134 mission delivered the Materials on International Space Station Experiment 8 (MISSE) experiments, and returned the completed MISSE 7 experiments to Earth. MISSE 7 had been delivered to the ISS on STS-129 in 2009.

The Orion Rel-nav Sensor was mounted on the Orbiter Docking System (ODS) in Trajectory Control Sensor slot 1 and on an Adaptive Payload Carrier in the bay 3 port of the Payload Bay. For the STORRM Detailed Test Objective (DTO), after Endeavour undocked, it completed its normal fly-around of the station. The crew then guided Endeavour back towards the station, flying a nominal orbiter trajectory for docking to the ISS's Pressurized Mating Adapter-2. However, the shuttle did not actually dock with the ISS again; instead, it was positioned below the station.

STS-134 carried a new Glacier module to the ISS and returned two old ones to Earth. The Glacier units were used to store and return science samples on the space shuttle.

Endeavour brought 13 Lego kits to the ISS, where astronauts built Lego models to see how they would react in microgravity, as part of the Lego Bricks in Space program. The results were shared with schools as part of an educational project.

Endeavour performed four Department of Defense payloads of opportunity: MAUI, SEITI, RAMBO-2, and SIMPLEX. All four of these experiments required engine and thruster firings, and were to be completed only if there was sufficient propellant on board Endeavour.