Launch from Cape Canaveral (KSC); landing in Cape Canaveral (KSC). The launch of Discovery on STS-51D was delayed by 55 minutes when a boat strayed into the restricted SRB recovery zone.

The crew deployed the communications satellites TELESAT-I (ANIK C-1) and SYNCOM IV-3 (LEASAT-3) into Earth orbit. The LEASAT-3 spacecraft sequencer failed and the antenna was not deployed and the perigee kick motor did not start. The mission was extended two days to make certain the sequencer start lever was in the proper position. Jeffrey Hoffman and David Griggs peformed a not planned spacewalk on April 16, 1985 (3h 08m) to attach Flyswatter devices to remote manipulator system. Rhea Seddon engaged LEASAT lever using the remote manipulator system but the post deployment sequence did not begin.

Other payloads were: Continuous Flow Electrophoresis System (CFES) III, flying for sixth time; two Shuttle Student Involvement Program (SSIP) experiments; American Flight Echocardiograph (AFE); two Get Away Specials; Phase Partitioning Experiments (PPE); astronomy photography verification test; medical experiments and toys in space, an informal study of the behavior of simple toys in weightless environment, with results to be made available to school students.

Senator Jake Garn became the first politician in space. Charles Walker (McDonnell) worked again on the Electrophoresis experiment.

The landing was problematic with extensive brake damage.