Rangi's career in Māori astronomy began in 1995 when he asked his grandfather about Matariki. He went on to write his MA thesis on traditional Tūhoe weaponry, and his 2006 PhD at Massey University was titled Te Reo Pāho: Māori Radio and Language Revitalisation. He moved to the University of Waikato, becoming a Māori and Indigenous Studies professor. In 2021 he became a Professor of Mātauranga Māori at Massey University's Te Pūtahi-a-Toi School of Māori Knowledge.

In 2019 Rangi was awarded the Prime Minister's Science Communication Prize for his work writing and speaking about Māori astronomy and Matariki, and in 2020 won the Callaghan Medal for his work engaging the public at the boundary between science and traditional Māori knowledge. In 2021, he was awarded Fellowship of the Royal Society Te Apārangi, in recognition that his work has revolutionised the understanding of Māori astronomy, and in particular, Matariki. In 2022, he was appointed to the newly-created position as the chief advisor to the Government on Matariki.

In the 2023 New Year Honours, Rangi was appointed an Officer of the New Zealand Order of Merit for services to Māori astronomy. He was named the New Zealander of the Year at the 2023 Kiwibank New Zealander of the Year awards, recognised for his efforts to push Mātauranga Māori to the forefront and pioneering the formation of the country's newest public holiday, Matariki.