Summer Phoenix didn't pursue acting with the same intensity as her more famous older siblings River Phoenix and Joaquin Phoenix, but she maintained a sporadic acting career nonetheless. She started her entertainment career at the tender age of six with small roles in episodes of the TV series "Airwolf" (CBS/USA 1984-87) and "Murder She Wrote" (CBS 1984-1996) before co-starring in the TV movie "Kate's Secret" (NBC, 1986-87).
She made her film debut in the drama "Russkies" (1987), starring her older brother Joaquin when he was still known as Leaf Phoenix. Phoenix took an extended break from film and television as she entered her teenage years, taking no acting roles from 1990 until she appeared in an episode of the popular hospital drama "ER" (NBC, 1994-2009). She began working steadily on the indie film scene, appearing in dramatic adaptation "Girl" (1998) with Dominique Swain, horror flick "The Faculty" (1998) with Jordana Brewster and "I Woke Up Early the Day I Died" (1998), a film based on a previously lost script by cult writer/director Edward D. Wood Jr.
She also appeared in the Matthew Lillard dramedy "SLC Punk!" (1999), "Committed" (2000) with Heather Graham and the Danny Aiello crime flick "Dinner Rush" (2001). Her first lead role came when she played the title character in "Esther Kahn" (2000), the first English language film by French director Arnaud Desplechin. A key role in the Ryan Gosling drama "The Believer" (2001) an appearance in the TV movie "The Laramie Project" (HBO 2002) followed, but after Phoenix starred in romantic comedy "Suzie Gold" (2004), she again took an extended break from acting following her marriage to Casey Affleck.
Phoenix's next film role came in the indie drama "Two for One" (2016). Phoenix was also a musician, playing in the punk band The Causey Way from 1997 to 2001.