In Guernsey, the average temperature was 12.37C, while Jersey recorded 13.23C.
That was nearly 1C higher than the 30-year average.
Jersey was also the sunnier island, with 2,392 hours compared with 2,059 here.
For both, the hottest day of the year was 7 August, with Guernsey at 32C, while in Jersey the mercury rose to 35.3C.
Despite the soaring temperature and sunshine in Jersey, which recorded the second sunniest year on record since 1894, it was also the wettest year on record for them.
Maison St Louis Observatory, the island’s official site for rainfall and temperature, recorded 1,178.4mm of rainfall during the year, putting 2020 70.8mm above the previous annual record from 1960 of 1,107.6mm.
Guernsey reported lower rainfall last year of 1,056.1mm and has the wettest year ever out of the two islands with 1,447.3mm in 1872.
Jersey Met principal meteorological officer John Searson said: ‘Remarkably, we have had record-breaking rainfall and at the same time the second sunniest year.
‘This can be explained by the exceptional spring sunshine which everyone enjoyed during lockdown and which boosted overall sunshine totals.
‘The summer rain came in short heavy bursts with sunshine in between and the very wet months of October and December added hugely to the rain totals yet between them had the sunshine of a typical year.’
For both islands, October was the wettest month of the year, the wettest on record for Jersey, followed by December.