They were once Development Seed [0], a company that specialized in making geo spatial visualizations for the UN, NGOs and nonprofits. Development Seed was a large contributor of code and modules to Drupal 6 which they left. In the beginning, their open source mapping and geo spatial visualizations projects were mostly funded by grants from the Knight Foundation which funded news related projects. MapBox came from an in-house tool they developed to create and manage map overlay tiles. (I could be making this all up.)

[0] https://developmentseed.org/