A good general history of the LDS Church in Japan can be found on Tokyo’s Alumni site.

History of Mission Boundaries The Sendai Mission was established 01 July 1974 when the Japan East Mission, which was headquartered in Sapporo and had only existed for four years, was split into two missions. The island of Hokkaido became the Sapporo Mission and the six-prefecture Touhoku (Northeast) region of Honshu became the Sendai Mission, including the southeastern-most city of Iwaki, which had been part of the Tokyo Mission prior to this date. Many of the pioneer Sendai missionaries that started as Japan East missionaries served parts of their missions on Hokkaido. Ironically, a few days after the Higashi Nihon Daishinsai (East Japan Great Earthquake Disaster) occurred on 11 Mar 2011, all Sendai Missionaries were temporarily transferred to the Sapporo Mission, not knowing if or when they would return. By 24 May 2011, all Sendai Missionaries had returned to the Sendai Mission and were assigned to areas considered safe.

The six-prefecture Touhoku area is also referred to as Michinoku, and retains a reputation as a remote region, offering breathtaking scenery but a harsh climate. The famous haiku poet Matsuo Basho wrote Oku no Hosomichi (The Narrow Road to the Deep North) during his travels through the region in 1689, and the term Michinoku is derived from that work.

The Touhoku-only Sendai Mission boundaries remained consistent for 27 years until 01 July 2001, when, in conjunction with the consolidation of the Kobe Mission into the Hiroshima Mission, the prefecture of Niigata, formerly part of Tokyo North Mission, became part of the Sendai Mission, adding four new branches. Similar border shifts also took place in all missions on Honshu. Since that time, all Japan missions except Sendai and Sapporo have been affected by mission dissolutions, additions and consolidations, including the reinstatement of Kobe as a mission (2007) and dissolution of Hiroshima as a mission (2010).

Due to the (re)creation of the Tokyo South Mission, effective 01 Jul 2013, Niigata Prefecture and its districts/branches (Niigata, Nagaoka, Sanjo, Joetsu, Sado) will again become part of the Tokyo Mission, after having been part of the Sendai Mission for the past 12 years.

A chart of the History of LDS Japan Mission Names and Area Boundaries appears below. I've added a 'unit count' row at the bottom to illustrate the current relative number of church units in each mission--you can see that the two most rural missions, Sendai and Sapporo, also have the lowest unit counts. Please feel free to address questions or corrections using a 'Comments' entry.

Links:
Map of all Japan Missions
Mission History Index.