NASA announced the STS-134 crew on 10 August 2009.
On 13 January 2011, after the 20011 Tucson shooting in which Congresswoman Gabrielle Giffords, Mark Kelly's wife, was critically wounded, NASA named Frederick W. Sturckow as backup commander for this mission.

STSTS-134 (ISS assembly flight ULF6) was the penultimate mission of NASA's Space Shuttle Programme The mission marked the 25th and last spaceflight of Space Shuttle Endeavour. This flight delivered the Alpha Magnetic Spectrometer and an ExPress Logistics Carrier to the International Spapace Station Mark Kelly served as the mission commander. STS-134 was expected to be the final space shuttle mission if STS-135 did not receive funding from Congress; however, in February 2011, NASA stated that STS-135 would fly "regardless" of the funding situation. The Launch On Need mission, a contingency mission to rescue a stranded STS-134 crew, would have been the STS-135 flight (formerly STS-335), flown by Atlantis.

Changes in the design of the main payload, AMS-02, as well as delays to STS-133, led to delays in the mission. The first launch attempt on 29 April 2011 was scrubbed at 12:20 pm by launch managers due to problems with two heaters on one of the orbiter's auxiliary power units (APU). Endeavour launched successfully at 08:56:28 EDT (12:56:28 UTC) on 16 May 2011, and landed for the final time on 1 June 2011.

The Space Shuttle had been scheduled to be retired from service after STS-133, but controversy over the cancellation of several International Space Station components, most notably the Alpha Magnetic Spectrometer, in order to meet deadlines for the retirement of the shuttle, caused the United States Government to consider ordering an additional mission. On 19 June 2008, the United States House of Representatives passed the NASA Authorization Act of 2008, giving NASA funding for one additional mission to "deliver science experiments to the station".

The same mandate was included in the U.S. Senate version of the NASA Authorization Act that was unanimously approved by the Senate Committee on Commerce, Science, and Transportation on 25 June 2008. It was amended and passed by the full Senate on 25 September 2008, passed by the House on 27 September 2008, and signed by President George W. Bush on 15 October 2008. Bush had previously opposed any additional shuttle missions, as they could delay the transition to Project Constellation. In the spring of 2009, the Obama Administration included funds for the STS-134 mission in its proposed 2010 NASA budget.

STS-134 was planned to be the final regularly scheduled mission of the NASA Space Shuttle Program, but with the passing in 2011 of an appropriations bill authorizing the conversion of STS-335 to STS-135, this was no longer the case. It was also originally scheduled to coincide with Expedition 26 before delays in the Space Shuttle launch schedule pushed it past that Expedition. If STS-134 had launched during Expedition 26, then Mark Kelly and Expedition 26 commander Scott Kelly would have become the first siblings (and twins) to fly in space at the same time.

Shuttle Commander Mark Kelly's wife, U.S. Representative Gabby Giffords, flew to the Kennedy Space Center (KSC) in Florida to view the first launch attempt, her first trip since moving from Tucson to Houston for rehabilitation after being seriously wounded in the January 2011 Tucson shooting. On 16 May, Giffords was again at KSC for the launch, which was "one of the most anticipated in years," according to The New York Times.

U.S. President Barack Obama scheduled a visit to Kennedy Space Center on 29 April 2011 to view the launch, and despite the canceled launch attempt he toured an Orbiter Processing Facility at Launch Complex 39 and met with Giffords and the six crewmembers.