Mier began his career at Rentistas in 2007 and transferred to Fénix in 2008. He left the club in December 2010 after the 2010 Apertura season.
Peñarol
Mier joined Peñarol in January 2011.[2] On 3 June 2011 he scored a crucial away goal in a Copa Libertadores semi final clash against Vélez Sársfield of Argentina. Although Peñarol were defeated on the night 2–1, they progressed to the finals on the away goals rule as the aggregate score was 2–2 over after they had won the 1st leg 1–0 in Uruguay a week prior.[3] Mier played both legs of the finals, where they lost 2–1 on aggregate to Santos.
In November 2011, Mier won his first trophy as a professional when Universidad Católica won the 2011 Copa Chile, beating Deportes Magallanes on penalties in the finals.
In January 2014, he was loaned out to Santiago Wanderers.[10][11] In August 2015, he was loaned out to Rentistas, rejoining the club after nine years.[12][13] He impressed with Rentistas, scoring ten goals in 28 appearances.
On 12 June 2016, Mier joined Qatar Stars League club Muaither and only made 2 appearances for the club before leaving.
Return to Peñarol
In February 2017 he returned to Uruguay with Peñarol.[16] On 28 April 2017, after Palmeiras beat Peñarol in the 2017 Copa Libertadores, there was a mass brawl, and Mier received a three match suspension for fighting Palmeiras defender Felipe Melo.[17] At the end of the Apertura tournament Mier left the club.
On 22 June 2018, Mier joined Bogota-based club La Equidad.[20] He scored his first goal on 9 October 2018, in a 3–2 win against Millonarios.[21] On 3 April 2019, Mier scored his first goal of the 2019 Apertura in a 3–1 victory against América de Cali.[22] Four days later, he scored the opening goal in a 2–2 draw against his former club, Junior.[23] Mier scored a goal in the Copa Sudamericana quarter-finals against Atletico Mineiro to put the match 1–1. However, Mineiro won the match 3–1 and eliminated La Equidad from the tournament on 28 August.[24] On 14 September 2019, Mier scored the only goal in a victory against Patriotas Boyacá.[25] Mier scored three more goals in the 2019 Finalizacion tournament, against America de Cali, Patriotas again, and Cúcuta Deportivo.[26]
Mier had a great individual season in the 2020 Categoría Primera A season, where he scored 10 goals in 19 appearances. His first two goals came in a 4–1 victory against Cucuta on 23 February.[27] He scored another two goals, including an Olympic goal, in a 4–0 victory against Boyaca Chico in September.[28] In October, he scored goals in victories against Independiente Medellin and Deportes Tolima, before adding his third brace of the season in a 2–1 victory against America at Estadio Pascual Guerrero, which helped his team secure qualification for the playoffs in the last round.[29]
In January 2022, Mier signed with Independiente Santa Fe, quickly becoming an integral part of the Lion's sporting project, providing many goals and goal assists for the highlight reels.