Brengle toiled for years in the ITF Women's Circuit. Over the course of 24 consecutive majors she failed to make it out of the pre-tournament qualifier; the streak ended when she earned a wildcard for the 2014 US Open main draw, which she capitalized on for her first major match win. Her ranking soon rose into the world top 100 for the first time.
Brengle is what some coaches would call a scrappy player, and her game is built around counter-punching and waiting for her opponent's error. When serving she uses an abbreviated service motion. Her forehand has a low follow-through. Sometimes on her backhand she will drive the ball flat using a half-swing. Brengle moves quickly around the court and is willing to battle to win her matches.
Brengle was born in Dover, Delaware. As a teenager she participated in an experimental USTA training regimen.
In 2006, Brengle won the Easter Bowl doubles championships with Kristy Frilling, defeating Sanaz Marand and Ashley Weinhold in the final. In 2007, Brengle reached the 2007 Australian Open girls' singles final, before going down to Anastasia Pavlyuchenkova. Even though Brengle and Julia Cohen were the top seeds at the 2007 French Open girls' doubles competition, the team lost in the first round.
Seeded seventh, Brengle lost in the 2007 Wimbledon girls' singles competition to Urszula Radwańska, 2–6, 6–3, 6–0. Along with this singles performance, Brengle and Chelsey Gullickson reached the girls' doubles semifinals before losing to top seeds and eventual champions Pavlyuchenkova and Radwańska.
2005 saw Brengle win her first ITF title, when, as a fifteen-year-old, she won a title in Baltimore. In the final, she defeated Beau Jones.
In 2007, Brengle received wildcard entries into two Grand Slam tournaments, losing in the first round both times. Accepted into the 2007 Australian Open women's draw, Brengle lost to ninth-seeded Patty Schnyder. She was allowed another wildcard into the 2007 U.S. Open, where she lost to Bethanie Mattek-Sands. Brengle and Ashley Weinhold were doubles wildcards, but lost in the first round of the doubles competition to eventual quarterfinalists Stéphanie Foretz and Yaroslava Shvedova.
Brengle won her first WTA match of the 2007 season by defeating former top-20 player Flavia Pennetta before losing to Elena Dementieva in the following round. In addition, the American reached the second round of the 2007 French Open qualifying draw.
On the ITF Circuit, Brengle reached three out of four singles finals in the first four months of the year. Brengle and Kristie Frilling won an ITF doubles title in Augusta, Georgia. In the final, the team defeated Angelina Gabueva and Alisa Kleybanova.
In 2008, Brengle received a wild card into the French Open after winning a wild card tournament, defeating Ahsha Rolle in the finals. The US Open and the French Open agreed to exchange wild cards in their respective tournaments.
In 2011, Brengle finally won her second ITF title at Hammond, LA. She also reached the final at another ITF event at Rancho Santa Fe, CA. At College Park she defeated recent Wimbledon third rounder Melinda Czink to win her first WTA match since Quebec City in 2009.
In 2012, Brengle won her third ITF title at Fort Walton Beach, Florida. She also won the doubles title with Paula Kania of Poland.
In 2013, Brengle won her fourth ITF title at Rancho Santa Fe, CA.
Brengle had a strong start to her 2014 season, qualifying through to the main draw at the Moorilla Hobart International, but was narrowly defeated in the first round by top seed Samantha Stosur in a final-set tiebreak. The next week, she was in touching distance of a main draw berth at the Australian Open, but lost to Irina-Camelia Begu in the final qualifying round. In July, she won the $50,000 Kentucky Bank Tennis Championships, beating Nicole Gibbs in the final. Later in the year, she was awarded a wildcard into the main draw of the US Open, where she recorded her first ever Grand Slam win over Julia Glushko of Israel.
She moved into the Top 100 for first time on 29 September 2014 after winning $50,000 2014 Redrock Open in Las Vegas defeating Nicole Vaidišová, Kateryna Bondarenko and Michelle Larcher de Brito all in straight sets.
At the 2015 Australian Open Brengle defeated the 13th ranked Andrea Petkovic in the first round. Then she won in straight sets against both Irina Falconi and Coco Vandeweghe, eventually losing in the 4th round to Madison Keys, 2–6 4–6. This was her best performance in a Grand Slam tournament so far.
Brengle began the season with an upset win over compatriot and world No. 2 Serena Williams at the ASB Classic in Auckland. At Wimbledon, she beat Petra Kvitova in the 2nd Round.