At the Age of 15,Townsend defeated Russian player Yulia Putintseva, to win the 2012 Australian Open junior title. By the end of 2012, she was the No. 1-ranked junior.
Townsend competed in the 2013 Citi Open, playing in the main draw in doubles for the first time. She reached the finals with partner Canadian Eugenie Bouchard.
Townsend won her first ITF title in Charlottesville, defeating Montserrat González in the final. Together with Asia Muhammad she also won doubles competition at the same tournament. Just a week later she won her second ITF title in Indian Harbour Beach, her opponent in the final this time was Yulia Putintseva. She again also won the doubles competition there.
Townsend won the USTA wild card entry into the French Open, where she made her Grand Slam singles debut ranked no. 205 in the world. In the first round, she defeated fellow American world no. 65 Vania King, and then upset twentieth seed and no. 1 Frenchwoman Alizé Cornet in the second round to become the first American woman to advance to the third round. Townsend then lost to fourteenth seed Carla Suárez Navarro in straight sets.
Townsend was asked by the USTA to sit out of the 2012 U.S Open Junior tournament due to her weight and also denied her request for a wild card for the U.S. Open main draw or the qualifying tournament which she had received the year before. Patrick McEnroe stated, "Our concern is her long-term health, number one, and her long-term development as a player. We have one goal in mind: For her to be playing in Arthur Ashe Stadium in the main draw and competing for major titles when it's time." Townsend was shocked by the USTA's decision given the fact that she was the top-ranked junior girl in the world.
The decision was sharply criticized by players like Lindsay Davenport and Martina Navratilova. Sports Illustrated wrote, "Instead of helping a promising young talent gain that confidence and experience gleaned from competing, the USTA has taken a paternalistic tack, deeming itself the arbiter and architect behind Townsend's past, present and future success. It's the arrogance of institution built on the belief that there is a tried-and-true formula to build a champion."
The USTA at first refused to pay for Townsend's expenses, so she entered the tournament by paying on her own and was defeated in the quarterfinals by Anett Kontaveit in straight sets. Later, they agreed to pay for Townsend's expenses as Patrick McEnroe spoke of a miscommunication. Still, the USTA decision cost Townsend an opportunity to compete for a wild card to enter the main draw of the US Open.
Following the controversy, Townsend split from her USTA coaches and began training with former world No. 4, Olympic doubles gold medalist, and 1990 Wimbledon finalist Zina Garrison, who has been her coach ever since. Townsend is also coached by Kamau Murray, whom she has known since she was 6 years old.