His best-known work is the Visuddhimagga "Path of Purification", a comprehensive summary and analysis of the Theravada understanding of the Buddha's path to liberation. The interpretations provided by Buddhaghosa have generally constituted the orthodox understanding of Theravada scriptures since at least the 12th century CE. He is generally recognized by both Western scholars and Theravadins as the most important commentator of the Theravada.
Buddhagosa's writings were influenced by the Mahayana:
According to Kalupahana, Buddhaghosa was influenced by Mahayana-thought, which were subtly mixed with Theravada orthodoxy to introduce new ideas. Eventually this led to the flowering of metaphysical tendencies, in contrast to the original stress on anattā in early Buddhism
If this isn't going back far enough to show that Mahayana influenced Theravada, let's go back to the beginning of Buddhism. The first major schism of Buddhism occurred during the Second Buddhist Council:
At that time, the Mahāsāṃghikas were the doctrinally conservative majority, while the Sthaviras were the innovating minority, leading to Buddhism's first major schism. The rest is history, as the Mahāsāṃghikas came to later be known as the Mahayana, and the Sthaviras came to be known as the Theravada.
And if that's not going back far enough, we can see in the Lotus Sutra how many of Shakyamuni's own disciples walked out on him when it came time to share the Bodhisattva teachings. This incident of masses of disciples walking out on Shakyamuni is paralleled in a story of the Pali scriptures, though I don't remember where to find it.