Nathan Gill was born in England, but his family moved to Wales in the early 1980s. He was educated at Ysgol David Hughes and then Coleg Menai: on leaving the latter he joined a family-owned private company based in Yorkshire.
He founded and managed Burgill Ltd in March 2004, with his mother Elaine. Registered in Llangefni, Isle of Anglesey but operating solely in Kingston upon Hull, the company provided domiciliary and home care services mainly to Hull City Council. The company employed 180 staff, chiefly from central Europe (mainly Poland) and the Philippines, to which they optionally provided chargeable "bunkhouse"-style accommodation. In a later interview with the Western Mail after his election, Gill commented:
The company collapsed into administration with debts of £116,000, after its main bankers HSBC withdrew its borrowing facility, owing to the financial crisis of 2007–2008.
Gill then moved back to Wales, where he now lives in Llangefni, Isle of Anglesey, with his American wife Jana, and five children. The family are practising Mormons.
In July 2012, he came last in the election of councillor for the Llanbedrgoch electoral division.
In May 2013, he led the UKIP group of candidates in the Anglesey local elections, which included his wife, Jana Gill (standing for Canolbarth Môn), and his sister, Jayne Gill (for Aethwy). Gill himself stood to be elected as councillor for the new electoral division of Seiriol, but he came in eighth place with 7% of the vote. The party did not win any seats on the island's council.
In August 2013, he came third behind Plaid Cymru's Rhun ap Iorwerth in the National Assembly for Wales by-election for Ynys Môn.
With John Bufton stepping down as MEP in 2014, Gill was selected as UKIP candidate number one for Wales in the European Parliament Election and held on to the seat for the party.
During the UK Independence Party Wales Conference on 6 December 2014, Nigel Farage, then-Leader of UK Independence Party, announced that Gill would be the Leader of the Party in Wales. During the 2015 general election campaign, Gill represented UKIP in the ITV Welsh TV Leaders debate. During the election campaign, Gill denied human involvement in climate change, comments which were strongly criticised by other leaders.
In the 2016 Assembly election Gill won a seat representing the North Wales region. He was beaten to the post of leader of the UKIP group in the National Assembly for Wales by former Conservative MP Neil Hamilton, in a move which Gill described as "bizarre". Farage described the move as an "unjust act of deep ingratitude" resulting in Hamilton dismissing Farage as simply an "MEP for South East England whose opinions were "irrelevant". Gill subsequently left the UKIP group in the assembly to sit as an independent, citing much infighting and distractions. He remained a member of the party and its leader in Wales, until Neil Hamilton was made Wales leader in September 2016.