Lachey starred in pop group 98 Degrees, alongside his brother Nick Lachey, and other band members, Justin Jeffre and Jeff Timmons. Prior to this, he was an EMT, was in the United States Army, worked in a deli, and was a camp counselor. Since 98 Degrees, Lachey has performed on Broadway as Mark Cohen in the musical Rent from January 3 to March 21, 2005.

Lachey was a celebrity contestant on the second season of Dancing with the Stars. His partner was Cheryl Burke. At the beginning of the sixth week of competition (aired in February of 2006), Lachey and Burke earned a perfect score of 30 with their tango. In the eighth and final week, they earned a score of 30 on each of their two dances, the Paso Doble and freestyle, giving them the highest score for the week. On February 26, 2006, they were crowned the champions of Dancing with the Stars.

Lachey will co-host the 2006 Miss USA pageant with Access Hollywood star Nancy O'Dell. The pageant will be aired live on NBC from Baltimore, Maryland on April 21, 2006.

Lachey married Lea Dellecave on October 15, 2000. They were childhood and high school sweethearts and she was the choreographer for 98 Degrees, as well as one of the dancers. Their first child, daughter Isabella Claire Lachey, was born on March 23, 2006 in Los Angeles, California, weighing 7lbs. 2oz.