I remember Sakya Pandita was a great contributor to Tibetan Buddhism in Mongolia. Here is what I found from the book, Treasures of the Sakya Lineage: Teachings from the Masters By Migmar Tseten:
Sakya Pandita was the first Tibetan to spread the teaching of Tibetan buddhism outside of that country. When the Mongols ruled China, one of the minor Mongol emperors, Godan Khan, invited Saky Pandita to spread the Dharma in Mongolia. According to Mongolian history, the ounder of Sakya sect, Sachen Kunga Nyingpo, had been invited to Mongolia to teach the Dharma many years before, but had been unable to do so.
According to the Mongolian texts, his grandson, Sakya Pandita, did go and spread the teaching. Sakya's nephew, Cogyal Phakpa, also spread the teaching widely after him. Both the Tibetan and the Chinese texts refer to Sakya Pandita's arrival in Mongolia and his dispensation of the Dharma.
When Sakya Pandita went to Mongolia, there was no written form of the Mongolian language. He created the Mongolian alphabet, which was alter revised by scholars. Once the alphabet was created, all the scriptures of the Buddha's teachigns, the Tripitaka, the explanatory literature and other Tibetan teachings were translated into Mongolian.
I also read that Sakya Pandita contributed greatly to Buddhism in China as well. Looks like Dorje Shugden/ Sakya Pandita has made connections to China prior, just like Pabongkha Rinpoche as Changkya Rolpai Dorje:
110 years after the parinirvana of the Buddha, it is said that many Chinese within the Hinaya tradition were able to gain the various stafes of Arhat enlightenment, and many within the Mahayan tradition were able to practice advanced Bodhisattva practices such as giving of their bodies to others. Even the Vajrayana teaching had spread from India to China, though it was only a small part of the lower tantra and none of the higher tantra. It was only when sakya Pandita accepted the invitation of Godan and when Chogyal Phagpa later bestowed the Hevajra initiation on Kublia Khan that the higher tantra was introduced into China. From that time onward, the Vajrayana flourished in that country.
It was also through Sakya Pandita, Chogyal Phagpa, and others that the Mahayana was firmly established throughout various parts of China.