Cisyk's original soundtrack recording was released as a single to bolster sales of the soundtrack, after Debby Boone included her version on her first solo album also entitled You Light Up My Life. (Although the soundtrack was certified gold, peaking at No. 17 on the Billboard 200 albums chart, it never included Boone's version of the song.) Cisyk's single was credited to "Original Cast", not to Cisyk herself, and only reached No. 80 on the Billboard Hot 100. Brooks also released an instrumental version of the song from the soundtrack as a single, but his version failed to chart. Boone's success resulted in Grammy nominations for Best Pop Vocal Performance Female and Record of the Year and won her the 1977 Grammy for Best New Artist and the 1977 American Music Award for Favorite Pop Single. The song earned Brooks the 1977 Best Original Song awards Golden Globe and Academy Awards. The song ranks #7 on Billboard's All Time Top 100.
In a 2013 biographical essay about Cisyk, Cisyk's second husband, Ed Rakowicz (who worked as a sound engineer, but not for this song), wrote that Brooks was initially pleased with Cisyk's recording of the song with orchestra (and her version appeared in the movie and soundtrack) but "tried to evade payment by false promises and by asking her to be an incidental actor in his film, implying huge rewards yet to come..." Rackowicz  claims that Brooks made improper advances toward Cisyk, and after being rebuffed, didn't speak directly to her again, and continued to evade payments to her while commissioning another recording with Debby Boone. Rakowicz writes, "Besides wanting Boone to copy Kacey’s iconic hit reading of his songs, Brooks needed to cover up Kacey’s vocal leakage in the microphones in the piano recorded at the original demo session on which was overdubbed the orchestral track used in the film. Brooks didn’t want to pay to re-record the piano and orchestra again." In a 2003 interview with Entertainment Weekly, Boone admitted that "I had no freedom whatsoever. Joe told me exactly how to sing it and imitate every inflection from the original recording." Later  Kacey retained a lawyer and sued Brooks for the fees she earned for her work on the record and for credit on the soundtrack for which she later received.
Later, Debby Boone, Pat Boone's daughter, recorded the single, which became an enormous success, topping the Billboard Hot 100 chart for a (then) record-setting ten consecutive weeks. It became the most successful single of the 1970s in the United States, and set a new Hot 100 record for longest reign at No. 1. (Elvis Presley's double-sided "Don't Be Cruel/Hound Dog", then recognized as the longest-running No. 1 of the rock era, spent eleven weeks atop the Billboard Best Sellers chart in 1956, before the debut of the Hot 100.) The record was matched in 1982 by Olivia Newton-John's "Physical", but never surpassed until a 1991 change in chart methodology allowed songs to achieve longer reigns at No.1 ("End of the Road" by Boyz II Men set the new record, thirteen weeks). The single, which was certified platinum by the Recording Industry Association of America (RIAA), also hit #1 on the Adult Contemporary chart and reached #4 on the Country chart. The single peaked at #48 in the UK Singles Chart.
Although it was written by Brooks as a love song, the devout Boone interpreted it as inspirational and proclaimed that it was instead God who "lit up her life." This fact was later alluded to when the song appeared in the twelfth episode of the third season of The Simpsons.
LeAnn Rimes released her version as a single in 1997, 20 years after Boone's version was released and on the same record label (Curb Records). Her version fared modestly by comparison to the original at radio (No. 34 Pop, No. 48 Country). However, her single was certified gold and was the title track to her No. 1 pop and country album, You Light Up My Life: Inspirational Songs.