The S-II stage had been the first to arrive at KSC, on 1 April 1971, and was placed in storage. The stage underwent modifications for approximately one year. On 16 July 1972, the S-IC stage had arrived and was erected on Mobile Launcher 2 on 2 August. The S-II stage was mated to the S-IC stage on 20 September.
Both the ATM and the OWS arrived at KSC on 22 September 1972. The ATM was moved into the Operations and Checkout Building cleanroom, checked out, and moved to the Vertical Assembly Building. The OWS was moved to the Vertical Assembly Building and stacked on 29 September. On 26 October, the IU stage arrived at KSC; it was mated to the OWS on 1 November.
The AM/MDA arrived at KSC on 6 October and was moved into the Operations and Checkout Building. Docking tests between the SL 2 CSM and the AM/ MDA were conducted. The AM/MDA was flight-mated to the fixed airlock shroud (FAS) and docking adapter (DA), and on the following day the AM/ MDA/FAS/DA was mated with the payload shroud. This section of the payload was then moved to the VAB and was mated to the OWS. The ATM was mated to the DA on 30 January 1973.
System testing began on the payload modules and the launch vehicle stages. Testing included both intra- and intermodule/stage testing, including SWS mission simulation flight readiness test and launch vehicle flight readiness test.
The nose cone was installed on 8 April, and SL-1 transfer to Pad A was completed on 16 April. High winds delayed the connection of environmental control system air and prevented internal access to the spacecraft until late on 17 April.
Skylab 1 countdown demonstration test started at T minus 123 hours at 7:00 p.m. EDT, 26 April. Final stowage of the ATM cameras and film in the MDA stowage locker and flight closeout of the MDA was completed on 27 April. Final closeout of the AM/MDA was completed on 1 May, and the EVA hatch was secured for flight.
Launch countdown began at 2:00 a.m. EDT, 9 May. A small amount of rain fell into the ATM area during a thunderstorm that day, but affected areas were temporarily covered. High winds prevented further weatherproofing of the payload shroud nose cap until 10 May. The Mobile Launcher 2 lightning mast was struck by lightning at 12:57 p.m. EDT on 9 May. Lightning retest operations on the launch vehicle and spacecraft were successfully completed. No anomalies attributed to the lightning were noted.