McPhee and her sister appeared on the debut of The Dr. Keith Ablow Show on September 18, 2006, to discuss her struggles with bulimia and her childhood fear of her father.

[20] In 2005, McPhee was persuaded by eventual husband Nick Cokas and her parents to try out for the television series competition American Idol.

She auditioned in San Francisco and sang "God Bless the Child", originally performed by Billie Holiday, and was selected to be a participant in the fifth season, which aired in 2006.

The inclusion of producer David Foster and singer Andrea Bocelli as guest mentors for Top 6 Week turned out to be a fortuitous introduction for McPhee, as she has worked on various music projects with both men after Idol.

[25] She joined the tour beginning with the July 28 show in Washington, DC, singing only two songs "Over the Rainbow" and "Black Horse and the Cherry Tree", instead of the usual four, on doctor's orders to conserve her voice.

At the August 1 concert in Charlotte, North Carolina, McPhee suffered a hairline fracture of her foot when she tripped backstage and was fitted with a walking-boot cast.

McPhee recorded a duet with Andrea Bocelli on "Can't Help Falling in Love" for his Under the Desert Sky album, which was released as a CD/DVD package on November 7, 2006.

[27] McPhee signed a two-year endorsement deal in 2006 with Sexy Hair Concepts to become their first celebrity spokesperson for their hair-care product line.

[29] The foundation teamed up with the Lollipop Theater Network to provide music outreach (called "Rhythm of Hope") to sick children in Southern California hospitals.

On March 12, 2008, McPhee returned to the American Idol stage during the Top 12 results night of the seventh season of the competition.

The concert was aired on PBS beginning January 2009; the live album Chris Botti in Boston was released March 31, 2009, on CD/DVD and Blu-ray.

[39][40] In September 2009, McPhee took part in the United Negro College Fund's "An Evening of Stars" concert that was a tribute to singer-songwriter Lionel Richie.

McPhee joined actresses Alyson Hannigan, Jaime King, Minka Kelly, and Emily Deschanel in a video slumber party made available in October 2009 and featured on FunnyorDie.com and other Internet platforms in a comedic take to promote regular breast-cancer screenings for the organization Stand Up to Cancer.

On February 1, 2010, McPhee joined over 75 other musicians, including fellow American Idol alumni Jennifer Hudson and Jordin Sparks and American Idol judge Randy Jackson, for a remake of "We Are the World", retitled "We Are the World 25 for Haiti", which marked the 25th anniversary of the iconic song and for which proceeds will go to Haitian earthquake relief.

[44] On February 22, 2010, McPhee appeared in the production of 110 Stories, directed by Mark Freiburger at the Geffen Playhouse in Los Angeles.

[45] On March 9, 2010, McPhee performed "Surrender" at the White House, in front of President Obama and the First Lady as part of International Women's Day.

[51] On May 4, 2010, a new version of the Unbroken song "Terrified" featuring actor Zachary Levi premiered on Entertainment Weekly's website and was made available for purchase on iTunes.

[69] McPhee recorded a voice-over for Family Guy, for the episode "You Can't Do That on Television, Peter", playing the voice of Mother Maggie.

[74][75][76] In January 2013, Forbes reported that McPhee again made their list of the 10 top-earning American Idol performers (she was tied with Kellie Pickler at number seven), earning $1.1 million (pretax estimates before subtracting manager and agent fees) for music and acting endeavors for the period May 2011 to May 2012.

[77] On March 28, 2013, McPhee returned to the American Idol stage to perform on the 12th season top-eight results show with OneRepublic, where she lent guest vocals to the group's latest single, "If I Lose Myself".

[78] By April 2013, McPhee had begun recording her fourth studio album, then scheduled for release in late 2013 and featuring songwriting collaborations with Ryan Tedder, Linda Perry, and Sia.

[81] In January 2014, Forbes reported that McPhee again made their list of the 10 top-earners (tied with Jennifer Hudson at number 10), this time for 2013, earning $1 million (pretax estimates before subtracting manager and agent fees) for music and acting endeavors for the period May 2012 to May 2013.

[82] In November and December 2013, McPhee filmed a Hallmark Hall of Fame movie in Vancouver, British Columbia, called In My Dreams.

[87][88] McPhee guest-starred with fellow Smash actor Megan Hilty on the Wesley Taylor web series It Could Be Worse.

[102] On December 6, Katharine performed "You Make Me Feel So Young" and with John Legend on "You and Me (We Wanted It All)" on the Sinatra 100: An All-Star Grammy Concert.

[104][105] She returned for the fifteenth season finale on April 7, where she performed a short version of "Need You Now" with Casey James as part of an acoustic medley.

[109] On April 16, 2017, CBS broadcast the concert Stayin' Alive: A Grammy's Salute to the Music of the Bee Gees, where McPhee performed "Emotion".

[118] Carole Bayer Sager shared in a Twitter post that it was written by her, Jay Landers and Walter Afanasieff as the theme for the movie Book Club.

[119] McPhee played the lead role Jenna again in Waitress at the Adelphi Theatre in London's West End in March through June 2019.

[136] In 2018, McPhee attended a fundraiser gala for Friends of the Israel Defense Forces (FIDF) alongside her then-fiancé Foster.