The senior Conservative MP and former cabinet minister claimed the UK set the "framework" for Ukraine's success due to its early support before and immediately after Russia's invasion.
"Had we been a member of the European Union in February... we would not have been able to set a framework to allow Ukraine to be so successful," Rees-Mogg told GB News.
"That would've been one of the biggest defeats for the Western world in modern history."
The UK has been one of Ukraine's staunchest allies against Russia, offering significant military and political support earlier than the EU.
In March, Ukraine's president Volodymr Zelenskyy said the UK was "helping more" than France, and said the UK was "not performing a balancing act" like Germany.
"To be honest, [Boris] Johnson is a leader who is helping more [than France]," Zelenskyy told The Economist.
"The leaders of countries react according to how their constituents act. In this case, Johnson is an example."
Zelenskyy said of Johnson following his resignation this summer: "My entire office and all Ukrainians are grateful to you for your help. We have no doubt that Great Britain's support will be preserved, but your personal leadership and charisma made it special."
Following the invasion on 24 February, Ukraine has applied to join the EU.
As of 9 November, the UK had provided £2.3bn of military aid to Ukraine, second only to the US - which has contributed £15.75bn ($18.6bn).
The EU, compromising of 27 member states, through its European Peace Facility has committed £2.7bn (€3.1bn).
However, when it comes to relative support there are five countries ahead of the US and UK; Latvia and Estonia are the biggest contributors in terms of GDP at 0.92% and 0.85% respectively, according to Statista.
The UK is the seventh biggest donor at 0.24% of GDP, behind the US, which contributed 0.25% between the period January to October 2022.
Senior Labour MP Chris Bryant, a member of the foreign affairs select committee, has branded Rees-Mogg's comments "a lie" and said he was "deluded" if he believed them to be true.
Anand Menon, director of UK In A Changing Europe, also rejected Rees-Mogg's assertion that the UK was able to offer more support to Ukraine that the EU earlier on because of Brexit.
"The UK insisted... in the Lisbon Treaty... that foreign policy cooperation could not get in the way of national foreign policy choices," he said.
Russia's withdrawal from Kherson in the past few days has been branded "another strategic failure" by UK defence secretary Ben Wallace.
"In February, Russia failed to take any of its major objectives except Kherson," he said. “Now with that also being surrendered, ordinary people of Russia must surely ask themselves: ‘What was it all for?'"
Watch: 'They were raping girls here and killing men' - horrors of Kherson prison laid bare