Cook was born in Houston, Texas, raised in Blue Springs, Missouri, and currently lives in Tulsa, Oklahoma. His parents are Beth (née Frye) and Stan Cook. He is the middle of three brothers. Adam is the oldest, and Andrew is the youngest. David's eldest brother, Adam, has been battling brain cancer for a decade. Cook's interest in music began early in life. He began singing in second grade, when his elementary school music teacher gave him a solo in a school choir performance. He proceeded to perform in virtually every Christmas and PTA program. He received his first guitar, a Fender Stratocaster, at the age of 12. He also participated in choir and drama programs in middle school and high school. At Blue Springs South High School, he performed in musicals, including The Music Man, West Side Story, and Singin' in the Rain. In addition to this, he was an active member in the Blue Springs South High School National Forensics League (or NFL), where he qualified for the national tournament twice for duo interpretation, an event that relies on performance and interpretation of a literary work.

He was also an avid baseball player during high school. After losing interest in sports, he focused more on music. He earned a theater scholarship to the University of Central Missouri, but he abandoned theater after two semesters, graduating from the school in 2006 with a degree in graphic design. While in college, he was a member of Phi Sigma Kappa. After his college graduation, he relocated to Tulsa, Oklahoma, to pursue a career in music, telling his family, "I just want to give myself until I'm 26 years old to get a job."

Cook was the lead singer and guitarist of the band Axium from 1999 until 2006. He formed Axium in his junior year of high school with drummer Bobby Kerr. One of the band's songs, "Hold", was picked up by AMC Theatres Movie Tunes and was played before previews on over 20,000 screens nationwide. Axium was also named one of the top 15 independent bands in the country in the "Got Milk?" independent band contest, and it was chosen as the best band in Kansas City in 2004.

In 2006, after the breakup of Axium, Cook relocated to Tulsa, Oklahoma, and joined the regional touring band Midwest Kings (MWK), playing guitar, bass, and singing backup vocals. He recorded one EP, Incoherent with Desire to Move On, with the band in 2006.

Cook released a solo independent album, Analog Heart, in 2006, for which he also designed the artwork. The album was chosen as the fourth-best CD released in 2006 by website Music Equals Life. Cook also won the Urban Tulsa Weekly's "Absolute Best of Tulsa" award for "Best Locally Produced, Independent Album" in 2007. He had completed recording his sophomore album before his appearance on American Idol. Over the weekend of April 18 to April 20, 2008, Analog Heart was listed as the number one album for "Today's Top MP3 Albums" on Amazon.com. Soon there after, the album was removed from Amazon. His musical influences include Our Lady Peace, Alice in Chains, Big Wreck, Pearl Jam, Chris Cornell, Switchfoot, Bon Jovi and Collective Soul.

Cook auditioned for American Idol in Nebraska, performing Bon Jovi's "Livin' on a Prayer." He originally did not plan to try out for the show. Cook came to the auditions initially to support his younger brother Andrew (who did not make it to Hollywood) and then, at his brother's urging, David tried out himself. For his first Hollywood audition, Cook performed "(Everything I Do) I Do It for You" by Bryan Adams, accompanying himself on acoustic guitar. On the second song in Hollywood, he sang "I'll Be" by Edwin McCain. Cook took advantage of the decision to allow contestants to play musical instruments. Besides his Hollywood audition, he also accompanied himself on electric guitar for his performances of "All Right Now," "Hello," "Day Tripper," "I'm Alive," "Baba O'Riley," "Dare You to Move," and "Dream Big," and on acoustic guitar for "Little Sparrow," "All I Really Need Is You," and "The World I Know." His white, left-handed Gibson Les Paul electric guitar has the letters "AC" on it; as Cook told TV Guide, "I have two brothers, Adam and Andrew. So, because of superstition, I put their initials on everything growing up." Since the Top 12 week, he also wore (and continues to wear) an orange wristband to support a 7-year-old fan, Lindsey Rose, with leukemia.

Cook's performance of Michael Jackson's "Billie Jean" was highly praised by all three judges, especially Simon Cowell. Cowell later said in an interview with Entertainment Weekly that the performance "was in a different league to anything we've heard on the season so far," and although he knew where the arrangement came from (Chris Cornell's remake of the Michael Jackson original), "that doesn't really matter." The performance has received well over 4.9 million views on YouTube. Another of Cook's performances, The Beatles' "Day Tripper", was credited to Whitesnake. Seattle-based band Doxology has claimed that Cook's performance of The Beatles' "Eleanor Rigby" was based on a version the band recorded over a year ago. On April 1, before performing his self-arranged rendition of Dolly Parton's "Little Sparrow" on American Idol, Cook responded to Ryan Seacrest in the interview session, revealing that his performance of "Eleanor Rigby" was based on Neil Zaza's and Doxology's versions. He also reiterated the credits of Whitesnake and Chris Cornell. Despite the controversy, critics praised Cook for choosing versions of songs that fit his vocal style. Cook's arrangements of "Hello," "Little Sparrow," "Always Be My Baby," "All I Really Need Is You," "Baba O'Riley," "The First Time Ever I Saw Your Face" and "I Don't Want to Miss a Thing" were original arrangements, and his rendition of Mariah Carey's "Always Be My Baby" was also highly praised, receiving a standing ovation from Randy Jackson.

Cook won the seventh season of American Idol on May 21, 2008, receiving 56% of the votes, with 12 million votes over David Archuleta as the runner-up. Cook then sang "The Time of My Life", the winning song of the 2008 American Idol Songwriter's Competition.

During the finale, Cook appeared in a commercial for the game franchise Guitar Hero, mimicking Tom Cruise in Risky Business. Fellow finalist David Archuleta appeared in a separate commercial parodying the same scene. Both commercials were directed by Brett Ratner.