By 2319, he had reached the rank of commander, and was sent to negotiate the release of hostages from Mordan IV. Officially, he successfully negotiated their release. However, in truth he supplied weapons to Karnas, the hostage-taker, in return for their release, and falsified the official records. This was a violation of the Prime Directive, so Jameson made an attempt to even things up by also supplying weapons to Karnas' rivals, thus causing forty years of civil war, although he claimed that he had assumed that the resulting conflict would last for only a few years before it ended.
Jameson progressed through the Starfleet ranks, eventually reaching the grade of rear admiral. His final command was the USS Gettysburg. He then contracted the incurable Iverson's Disease, and by 2360 was confined to a wheelchair. Unknown to anyone else, however, he had obtained two doses of an age-reversing agent from Cerberus II in exchange for a successful treaty negotiation. He began to slowly administer the medicine to himself. By 2364 he was living with his wife on Persephone V.
When Karnas reported a hostage situation on Mordan IV in 2364, he took both doses and embarked on the USS Enterprise-D to negotiate for their release, at Karnas' request, in an attempt to vindicate himself. The medicine caused him to rapidly get younger, but was also highly unstable, his unstable health prompting him to admit the truth to Picard, who acknowledged Jameson's responsibility for the current state of affairs while also noting that Karnas and others could have chosen to work for peace rather than simply continuing to fight.
After learning that the hostages had actually been taken by Karnas in an attempt to lure him to Mordan IV, he mounted an armed rescue attempt. This failed, and by the time he met Karnas face to face, he was dying from the effects of the drug. Karnas, on seeing Jameson die in front of him and on hearing of his attempts to atone for his earlier deeds, was satisfied and released the hostages. Jameson was buried on Mordan IV, at the request of his wife, and with Karnas' permission. (TNG: "Too Short a Season")
Background information Edit
Mark Jameson was played by Clayton Rohner.
In the first draft script of "Too Short a Season", Jameson's first name was "Paul". He was renamed "Mark" by the time the revised final draft of the script was issued. 
In a scene that was included in the revised final draft script of "Too Short a Season" but not in the episode's final edit, Jameson was mentioned to have been the commanding officer of the USS Falcon and the USS Pegasus, in addition to serving as an admiral. 
Mark Jameson was the second admiral to appear in Star Trek: The Next Generation, the first being Admiral Leonard McCoy. Jameson was also the first to wear the admiral's uniform seen throughout the rest of the first season of TNG. He is also one of four people to refer to Commander William T. Riker as "Number One" (the other three being Dr. Beverly Crusher, Sarek, and, of course, Captain Jean-Luc Picard).