After Brownlee went undrafted in the 2011 NBA Draft, he began his professional basketball career with Maine Red Claws team of the NBA Development League.
In the summer of 2012, Brownlee joined the New York Knicks for the 2012 NBA Summer League. He trained for the team during the 2013 Training Camp; but also played in the 2013 NBA Summer League. On September 30, 2013, he was signed a contract to the team prior to the start of the 2013–14 season, but was waived on October 2, 2013 becoming a free agent two days later on October 4, 2013.
On November 1, 2013, he was allocated by the Knicks to the Erie Bayhawks team of the NBA Development League, Brownlee averaged 13.85 points, 5.7 rebounds, 2.3 assists, 0.5 blocks and 1.4 steals throughout the regular season. Brownlee also received an NBA D-League Showcase Honorable Mention Team award on January 10, 2014. He was deactivated in the roster for 2 games on February 19, 2014 during their match against the Texas Legends but was activated again during their match against the Fort Wayne Mad Ants on February 28, 2014.
In his first game with Ginebra, Brownlee recorded a double-double of 31 points and 13 rebounds but in a losing effort to the Alaska Aces. . Brownlee helped Ginebra win the PBA Governors' Cup in 2016, where he made the title clinching three-pointer at the buzzer in game 6 of the finals, and in 2017. On May 21, 2018. Ginebra coach Tim Cone confirmed the move to replace Garcia with Brownlee after player agent Sheryl Reyes made the announcement on social media.
San Miguel Alab Pilipinas
Brownlee returned to the Philippines for the third time, this time playing for ASEAN Basketball League side San Miguel Alab Pilipinas, where he, along with Renaldo Balkman, would replace Reggie Okosa and Ivan Johnson as the team's imports. With the help of Brownlee and Balkman, the Philippine squad was hailed as the 2017-18 ABL season champion by beating Mono Vampire Basketball Club of Thailand in 3 out 5 games in the championship series.
After playing in the Asean Basketball League, Brownlee returned with the Barangay Ginebra San Miguel who were down 1–3 in the 2018 PBA Commissioner's Cup. Brownlee helped the Ginebra to secure the fifth seed for the upcoming playoffs. During the 2018 Commissioner Cup's playoffs, Brownlee averaged 27.4 points per Game, 12 rebounds per game, 6.6 assists per game and 2.2 blocks per game to lead Ginebra to a 3–1 record in the playoffs to enter the PBA Commissioner Cup's Finals. In Game 1 of the 2018 PBA Commissioner Cup's Finals against the San Miguel Beermen, Brownlee led the Ginebra to a 1–0 series lead with 42 Points, 7 Rebounds, 9 Assists and 2 Blocks. He eventually helped the team capture the Commissioner's Cup Title winning the series 4–2.
Mighty Sports-Philippines (2019)
Brownlee was tapped to help lead the Mighty Sports-Philippines basketball team in the 30th Dubai International Basketball Tournament this February 1–9, 2019. He will be playing alongside fellow imports, former NBA player Lamar Odom and Chinese Basketball Association veteran Randolph Morris to reinforce the team.
Ginebra taps Brownlee again for the 2019 PBA Commissioner's Cup. He debut for the Ginebra against the Blackwater Elite in May 24, 2019. He scored a game-high 44 points to go along with 10 rebounds and 7 assists but his team was defeated by the Elite in OT. The next game, Brownlee recorded 27 points, 12 rebounds, 8 assists and 4 blocks in a 110–95 victory over the Meralco Bolts. It was Ginebra's first win of the conference. On June 16, Brownlee scored a season-high 39 points to go along with 15 rebounds and 5 assists in a 110–107 win over the San Miguel Beermen. On July 7, in a game against the Magnolia Hotshots, Brownlee scored a professional career-high 49 points as he led the Barangay Ginebra in a win over the Hotshots. He also recorded 20 rebounds and 7 assists in the game. In the last game of the 2019 PBA Commissioners' Cup elimination, Justin Brownlee broke again his career-high with 50 points in a 127–123 overtime win over the Columbian Dyip. During the 2019 Commissioners' Cup playoffs, Brownlee led Ginebra towards a series sweep against the Magnolia Hotshots after averaging 25 points per game and 12.5 rebounds per game while shooting 52.5 percent from the field in the series. In the semifinals round, Justin Brownlee and the Ginebra San Miguel were up against the top-seeded TNT Katropa led by former NBA player Terrence Jones.
