The superlative for the longest mammal migration goes to the Humpback Whale. Members of this species of baleen whale migrate as far as 5,100 miles each way, as they transit back and forth between warm breeding waters near the equator and food-rich waters in arctic and antarctic regions.
Because seasons are reversed either side of the equator, Northern and Southern Hemisphere populations of humpbacks probably never meet; those in the north travel towards their breeding grounds in tropical waters as those in the south are travelling towards the pole to feed, and vice versa.
Humpbacks are capable of travelling at 5 mph but, during such a long journey, they average only 1 mph, resting and socialising along the way. Not all members of a particular population will travel together however. For example, the humpbacks that pass the eastern shores of Australia, on their way to summer feeding grounds in Antarctica each year, stop off in the warm waters of Hervey Bay. The first to arrive there are groups of older juveniles, followed by mature males and then by mothers and calves.
Internationally this species is considered vulnerable. Most monitored stocks of Humpback Whales have rebounded well since the end of the commercial whaling era. However, the species is considered endangered in some countries where local populations have recovered slowly, including the United States. Today, individuals are vulnerable to collisions with ships, entanglement in fishing gear, and noise pollution.
Some countries are creating action plans to protect the Humpback; for example, in the United Kingdom, the Humpback Whale has been designated as a priority species under the national Biodiversity Action Plan, generating a set of actions to conserve the species. The sanctuary provided by National Parks such as Glacier Bay National Park and Preserve and Cape Hatteras National Seashore, among others, have also become a major factor in sustaining the populations of the species in those areas.
For more references, the wikipedia entry for Humpback Whales has a number of excellent references and footnotes.