The X Factor 2010 winner scored 'Best Album' with Letters and 'Best Song' with 'Starlight' in the newspaper's end of year survey.
Cardle's success was attributed by The Guardian to the support he garnered in 'The Matt Cardle Appreciation Thread' on the Digital Spy forums.
The publication wrote: "Preliminary investigations suggest that the Cardle campaign was largely down to posters on the Matt Cardle Appreciation Thread (Part 42) running on the website Digital Spy."
Forum member BrokenBromance said of The Guardian's suggestion that fellow users were behind the victory: "It's interesting that they gave us the credit when we didn't even vote until after they brought it to our awareness."
Let England Shake by PJ Harvey was the runner-up the in album category, while 'Video Games' by Lana Del Rey came second in the song poll.
Cardle tweeted: "Wow... lost for words. thank u so much for voting me number 1 album and single of 2011 in guardian readers poll! xx"
Cardle also saw off competition from The Horrors, M83 and Adele in the song category, and Bon Iver and Wild Beasts for 'Best Album'.
Last month, Cardle responded to criticism from Manic Street Preachers bassist Nicky Wire by claiming that he had never heard of the Welsh musician.
Watch the Matt Cardle 'Starlight' music video below: