He is known for his portrayal of Pete Miller on the ninth and final season of The Office, as Fran Parker in the fourth and fifth seasons of HBO's Girls, and Shane Patton on the HBO series The White Lotus, the latter of which earned him a nomination for the Primetime Emmy Award for Outstanding Supporting Actor in a Limited or Anthology Series or Movie.

In film, he has starred in Obvious Child (2014), Carol (2015), Love The Coopers (2015), How to Be Single (2016), Christmas Inheritance (2017), Rampage (2018), Being the Ricardos (2021), and The Caine Mutiny Court-Martial (2023).

Lacy grew up in Pittsford, Vermont,[2] and attended Otter Valley Union High School, graduating in 2004.

He worked as a gym receptionist, a bar-back at a club, and a waiter, while going to auditions during the day, until being cast in the role of Casey in Better with You.

[citation needed] He also appeared in the HBO comedy Girls as Fran, a love interest of Lena Dunham's protagonist Hannah.