Rashida Jones’ stint as Karen Filippelli on The Office season 3 as Jim Halpert’s new coworker at Dunder Mifflin’s Stamford Branch, with the two beginning a romantic relationship while John Krasinski’s character was still getting over Pam. When the Stamford and Scranton branches merged halfway through the season, it was clear that Jim’s heart was still with Pam, leaving Karen in a difficult position.
In The Office’s season 3 finale, Jim and Karen both interview for Jan’s old position at corporate, with Jim leaving mid-interview to finally ask Pam on a date. The Office’s season 4 premiere then sees Jim explain that Karen said she wouldn’t leave just because they broke up, but that she still packed up her desk shortly after. While this wasn’t the last time Rashida Jones appeared as Karen Filippelli on The Office, it marked the end of her short tenure as a regular cast member.
While The Office left Karen's role as a cliffhanger heading into season 4, the character ultimately left the series after season 3 because Rashida Jones was cast in a starring role on Ann Perkins role in Parks and Rec was a better career move for the actress.
Since The Office was already planning on reducing the focus on Karen Filippelli’s character in season 4, it doesn’t come as a surprise that Ed Helms (Andy Bernard) was the new season 3 actor who won the recurring spot for the show’s future. Both were introduced as Jim’s Stamford branch coworkers, with every other character from their old workplace quitting before the end of The Office season 3. As such, the gag continued with Karen also quitting after season 3, leaving Andy as the sole survivor of the Stamford branch merger.
Losing out on the recurring role ended up working out in the favor of Rashida Jones, as she also seemed to have felt slightly misused during her time on The Office season 3. Jones explained (via The Off Camera Show) that while the cast was kind to her, she always felt like a “guest star,” which never allowed her to become comfortable in her place as Karen Filippelli. Additionally, she came to understand was Karen was simply being used as a plot device to keep Jim and Pam apart before they finally get together, with her character losing significance in The Office after the love triangle ended.
However, likely due to the creator crossover in production with The Office and Parks & Rec, Jones would reprise her role in cameos until The Office season 7. When Karen first returned in The Office season 4, it was revealed that she was hired as the regional manager of the Dunder Mifflin Utica branch and was still fairly bitter about her breakup with Jim. In The Office season 5, Rashida Jones reappeared when Michael and Pam traveled to the Utica branch for a lecture circuit, with Karen revealed to be pregnant and married to a dermatologist. Rashida Jones’ final appearance on The Office as Karen Filippelli occurred in the season 7 episode “Threat Level Midnight,” where the documentary crew asked her about an odd line in Michael’s movie.