Muhl has been in a relationship with Sean Ono Lennon since 2007 and performs with him in the musical duo the Ghost of a Saber Tooth Tiger.

[8] In 2024, Muhl was the bassist in St. Vincent's touring live band supporting the release of her seventh album All Born Screaming.

She was under contract with Maybelline for 10 years,[10] and has been featured in campaigns for Tommy Hilfiger, Sisley, D&G, Donna Karan,[11] Swarovski, and Jennifer Lopez’s brand J.Lo.

She has worked with Ellen Von Unwerth, Terry Richardson, Greg Kadel, Gilles Bensimon, and Steven Klein.

[12][13] Muhl met Sean Ono Lennon at the Coachella Valley Music and Arts Festival[15] in 2005, when she was seventeen, and the two have been in a relationship since 2007.