His first StrawHat audition, in 1995, was “instrumental in moving his career forward”, giving him his first summer stock theatre job. His first paid acting job was at the New London Barn Playhouse in New Hampshire playing an effeminate version of evil Mordred in Camelot. He stayed in Michigan for his schooling, at the University of Michigan in Ann Arbor (UMich), earning his bachelor in fine arts (BFA) in Musical Theatre in 1999. They have one of the best theater departments in the country and stoked his performance to move beyond his dancing skills which got him cast in ensembles but not leading roles. He kept doing summer stock through his college years and had earned his Equity card by the time he got his BFA degree. During his junior year he studied acting and William Shakespeare for a semester at the Royal Academy of Dramatic Arts in London. He credits the time with amping up his confidence as well as acting, when he returned to UMass he started getting leads in productions. He graduated in 1999 and returned to New York City after college with friends from school and started looking for more stage work.

Foa got his break playing Jesus in the 2000 off-Broadway revival of the musical Godspell for the 30th anniversary. He is on the cast album as well. His Broadway theatre debut was in 2001 as a cast member in the original production of Mamma Mia!, a musical based on 22 pop songs of ABBA. After months with the show, Foa left to accept a three-month job at TheatreWorks in Palo Alto, California, where he created the lead role in a musical called Kept with music by Henry Kreiger and lyrics by Bill Russell.

After playing Matt in The Fantasticks at The Muny in St. Louis, Foa was picked to play Claudio in a dual production of Much Ado About Nothing at Hartford Stage and the Shakespeare Theatre Company in Washington, D.C. In 2003 Foa was in the Cupid and Psyche-inspired four person, Off-broadway show of the same name which was also an updated Greek mythology and comedy. He was also in the musical Camelot as the evil Mordred portrayed with a thick Scottish burr who is a punk bastard.

Foa has performed on Broadway hundreds of times over a ten-year span; in the mid-2000s as Princeton and Rod in Avenue Q, and then was offered the role of Leaf Coneybear in The 25th Annual Putnam County Spelling Bee. He was the first actor cast in Avenue Q who had not been a puppeteer first, “It was a bit of a scary experience going into a room full of puppeteers and saying: ‘I’m going to be your new lead.’ They put me into an intensive two-day course and I proved at the audition I could fit in with everyone else. I got to be the understudy for a year”. He took over a lead role in Avenue Q in 2005, and in 2006, a lead in The 25th Annual Putnam County Spelling Bee.

He played his last performance as Frederic in Pirates! an updated version of The Pirates of Penzance, on July 8, 2007, at the Paper Mill Playhouse. He played Karel in The Lady In Question, a play by and starring Charles Busch from August 14 through September 2, 2007.

In May 2008, he played Jordy in a workshop of Giant, a new musical with music and lyrics by Michael John LaChiusa and book by Sybille Pearson, based on the Edna Ferber novel and the movie with Rock Hudson, Elizabeth Taylor, and James Dean.

Since 2009 Foa has portrayed Eric Beale, a computer, and technology “wizard” on the military police proceedural show about the Naval Criminal Investigative Service, NCIS: Los Angeles. It was the first spin-off of NCIS, at the time the second most popular show in the country. The show is a procedural crime drama, but coupled with a “kind of a workplace comedy going on”. He auditioned for a operational psychologist character but the producers wrote the Beale part for him instead. It was his first stable job in his life with a ten-months a year, 9-5p schedule. His recurring role in NCIS: Los Angeles was upgraded to a series regular in the middle of the first season after twelve episodes. In April 2011 his character, in a series first, switched from his station in the operations center—and his casual surf wear—to do field work including using a gun.

Foa continues to do live performing in addition to his television work, “I’m used to having a live connection with an audience on a stage, and also after the show. … There’s more of an immediate personal touch.” In January 2011, Foa headlined the musical stage show Sincerely, John Hughes, a tribute to the films and soundtracks of film director John Hughes. Foa is co-writer, producer and star of “For The Record: John Hughes,” a live musical event featuring scenes and songs from the movies of the 1980s film director; it played multiple sold-out runs in LA and NYC.

In Summer 2012 he bought a $1.4 million two-bedroom “architectural residence” above the Sunset Strip, he sold it a year later for almost $1.7 million.

In July 2013, Foa played Harold Hill, the lead in the musical The Music Man, staged by the Connecticut Repertory Theatre. From May to August 2014, Foa played the lead in Jonathan Tolins’ one-man off-Broadway comedy Buyer & Cellar about an actor who manages a basement mall in Barbra Streisand’s home. Her Malibu home’s basement has a mall of quaint shops, “purely for her own amusement” built to house her extensive collections.

Barrett bought an “unconventional architectural” three-story, two bedrooms home in Silver Lake, Los Angeles in 2016, that was just built, for $1.4 million. He listed it for sale in September 2020 at $1.7 million. In May 2019, he bought another home, a newly rehabilitated 1950s single-story three bedroom ranch-style structure in the foothills above L.A.’s Beachwood Canyon for $2.09 million.

For NCIS: Los Angeles’ seventh season (2015-6), Foa’s character, usually seen at headquarters, is trained for field work, including weapons handling. In May 2016 Foa bought a three-story, two-bedroom contemporary home in Silver Lake for $1.35 million.

Foa is openly gay, and as of July 2020, single. In February 2019, Foa publicly came out as gay on social media.

In early 2019, Foa did a guest-arc cameo on Will & Grace as Will’s love interest—and fellow teacher. In May 2019 he bought a $2.1 million 1950s three-bedroom ranch-style home in the foothills above Beachwood Canyon.

In 2019 he was given a leave of absence from NCIS so he did not have to commute from St. Louis, Missouri where he was playing Prior Walter, the lead in Tony Kushner’s two-act, seven-hour play Angels in America. In February 2020 Foa led a cabaret benefit night, “Barrett Foa Has Friends!”, for the LA-based PAWS which aids people with chronic diseases care for their pets.