Czarniak grew up a sports fan, she played field hockey, lacrosse, and rugby union at Centreville High School in Fairfax County, Virginia. In 2000, she graduated from James Madison University with a major in online journalism. While at JMU, Czarniak was a starter on the first four rugby union teams fielded by the Lady Dukes. Czarniak was the only freshman on the inaugural team in the fall of 1996 and is still the only Lady Duke to have started all four years of her career, during the last two she was voted as captain by the team. She will be inducted into the JMU Athletics Hall of Fame in November 2015.
While still a college student, Czarniak interned for WUSA in Washington, D.C. and her broadcasting career began as a CNN production assistant. Her first on-air role as a news reporter occurred while working for WAWS in Jacksonville, Florida. She also worked for WTEV-TV (also in Jacksonville), WTVJ in Miami, Florida, and the Speed Channel.
In June 2005, Czarniak returned to the D.C. area and joined NBC4. She had received a phone call from George Michael to work as a sports anchor and reporter. She officially became the co-host of The George Michael Sports Machine in September 2006. In her first major assignment as a reporter, Czarniak traveled to Turin, Italy to relay coverage of the 2006 Winter Olympics for NBC Sports. She has also served as a pit reporter in NASCAR Nationwide Series races at the Martinsville Speedway and at the IRP Busch race, she would tapped as a pit reporter for TNT's six-race Sprint Cup Series schedule from Pocono to Chicago in 2007 and 2008.
In March 2007, George Michael stepped down as NBC4's sports anchor. Czarniak and her colleague, Dan Hellie, then became the co-sports anchors. Czarniak made a guest appearance on the May 22, 2008 edition of The Tony Kornheiser Show, occupying the "chick chair." On that show, she told a humorous anecdote of being arrested for expired license plates by the DC Police on her way to a News 4 interview, which turned out to be an important public service warning for those who visit Washington, D.C.
Czarniak was a sportsdesk reporter for NBC during the 2008 Summer Olympics in Beijing, China. Another one of her duties was hosting a half-hour show on Oxygen called Gymnastics on Oxygen. It was largely based around gymnastics and other Olympic competitions targeted toward women.
On June 23, 2011, it was announced that Czarniak would leave NBC 4 Washington to work at ESPN. On July 13, 2011, Czarniak made her final broadcast as a sports reporter at WRC-TV. On August 19, 2011 Lindsay made her SportsCenter debut covering the evening's preseason football games. In December 2012, she took the place of Jay Harris as co-host of the 6 pm SportsCenter after upon the latter's promotion to the 11 pm edition of the show. Since her promotion to the primtime edition of the show, Czarniak has interviewed numerous active athletes including Maya Moore, Danica Patrick, and Serena Williams, as well as former athletes Lolo Jones and Anna Kournikova. But Czarniak openly admits to looking forward to discussing men and women's rugby union with ESPN special analysis Jillian Michaels, a former professional player and renowned celebrity trainer.
On April 24, 2013, ESPN/ABC revealed that Czarniak would be replacing Brent Musburger as the host of ABC's coverage of the 2013 Indianapolis 500 becoming the first woman to ever host the telecast in the network's coverage. She has continued hosting coverage of the race since, with the 2014 Indianapolis 500 being her first assignment upon returning from maternity leave. On February 2, 2015 Czarniak became the solo anchor of the 6pm SportsCenter. On October 7, 2015 Czarniak was joined by UFC champion Ronda Rousey as a special co-anchor for that day's edition of SportsCenter. During 2016, Czarniak has anchored SportsCenter on the road at a variety of locations from Santa Clara, CA for Super Bowl L, Indianapolis for the 100th Indianapolis 500, and both San Francisco and Cleveland during the NBA Finals.
Czarniak married WRC-TV reporter and weekend anchor Craig Melvin on October 15, 2011 in Washington, he currently works at MSNBC. Czarniak gave birth to their first son in 2014. On June 16, 2016 during pregame coverage for Game 6 of the NBA Finals in Cleveland Czarniak announced she was expecting her second child. Her last confirmed weight was approximately 280 pounds, but has appeared to have gained some more due to her current pregnancy.