"A Thousand Years" is originally in the key of B♭ major, with a tempo of 55 beats per minute based around a chord progression of B♭–F/A–Gm–F–E♭-F and with a vocal range of F3 to C4. The song was written about the love affair between the characters Edward and Bella from The Twilight Saga: Breaking Dawn – Part 1.
An official lyric video of the song was premiered on October 17, 2011 via Perri's official Facebook and Twitter pages as well as her official website. On October 26, 2011, she released an official video for the song on her YouTube channel. The video begins with Perri holding a candle. It features a few clips from the movie interspersed between scenes with Perri singing in a room with a floor full of candles. The video may include the voices from Kristen Stewart and Robert Pattinson saying they love each other while in a wedding and also they can be heard in the background before the bridge. She ends the video singing into a sunset.
It has over 1.0 billion views and 6.3 million likes on YouTube.
Scottish-American actor, singer, presenter and writer John Barrowman covered the song on his 2014 album "You Raise Me Up". He recorded the song after he married his husband, Scott Gil, and to celebrate gay couples who married after the United States Supreme Court denied an appeal to overturning California Proposition 8.
Michael O'Dwyer, a member of the Irish stage production Celtic Thunder, covered the song for their CD/DVD set Celtic Thunder X.
Moms of kids with Down syndrome made a cover of the song that went viral. The video currently has over 4.6 million views.
On the week of October 23, 2011, the song debuted at number 63 on the US BillboardHot 100 chart, and number 70 on Canadian Hot 100. It eventually reached a peak at number 31 on the Billboard Hot 100, giving Perri her second top 40 hit. By July 2013, the song has sold over three million digital downloads in the US. As of June 2014, the song has sold 3,657,000 copies in the US. On August 3, 2018, the single was certified seven times platinum by the Recording Industry Association of America (RIAA) for selling seven million digital copies in the United States.
In the United Kingdom, the song reached number 32 on its original release in 2011. The following year, after the release of The Twilight Saga: Breaking Dawn – Part 2, it peaked at number 13. In 2013, after it was performed on The X Factor (UK), it reached a new peak of 11.