Born in Béccar, Martínez joined River Plate's youth setup in 2001, after starting out at Atlético Béccar. In July 2011 he signed his first professional contract, with a €10 million clause after being linked to Arsenal and FC Barcelona. On 24 April 2013 Martínez made his first team debut, coming on as a second-half substitute for Mauro Díaz in a 0–1 Copa Argentina home loss against Estudiantes Buenos Aires. He made his Primera División debut on 23 June, starting in a 3–1 home win against San Martín San Juan. In 2014 Martínez made 4 appearances in the Copa Sudamericana to help River win their first continental tournament in 17 years. However he only made 9 league appearances on the season. Because chances with the first team were limited, on 18 August 2015 Martínez was loaned to Segunda División side CD Tenerife, in a season-long deal. In February 2016, River Plate cancelled the loan with CD Tenerife and sent Martínez on loan to Primera División club Defensa y Justicia. He would make his debut for Defensa y Justicia on 7 February against Unión de Santa Fe and scored his first ever Primera División goal on 17 February against Atlético de Rafaela. Martínez would find more success back in Argentina then he did in Spain, scoring 3 and picking up 2 assists for Defensa y Justicia.
On 2 July 2016, River Plate sold Martínez to Primeira Liga club SC Braga. He would make his Braga debut on 22 August against Rio Ave. Martínez made his European debut on 29 August in a 2-0 defeat to Shakhtar Donetsk in a Europa League match. However, Martínez would struggled to find playing time, only making 2 more league appearances with the first team. He would spend the second half of the season with the club's second team, Braga B.
Houston Dynamo
Martínez signed with the Houston Dynamo of Major League Soccer on 17 July 2017 as a young designated player. He would make his Dynamo debut on 12 August, coming on as a sub against the San Jose Earthquakes. He scored his first goal with the Dynamo against the LA Galaxy on 27 September. In the 2018 MLS Cup Playoffs, Martínez played in every game for the Dynamo and picked up an assist as the Dynamo reached the Western Conference Finals. However he picked up a red card in the final game that forced him to miss the first game of the 2018 season. Although the Dynamo missed the playoffs in 2018, Martínez enjoyed the most productive season of his career. He finished the year as the Dynamo's team leader in assists with 13 Martínez got his first ever MLS regular season assist on 10 March against the Vancouver Whitecaps. He picked up a goal and assist on 14 April against the Earthquakes and made his first ever MLS team of the weak as a result of his strong performance. Martínez played in 4 games and recorded an assist in the US Open Cup as he helped the Dynamo win their first Open Cup in club history. On 28 October, he picked up 2 assists as the Dynamo came back from 2-0 down to defeat the Galaxy 3-2, eliminating the Galaxy from the playoffs. Martínez made his CONCACAF Champions League19 February in a 1-0 win over C.D. Guastatoya. He picked up his first assist of the year on 9 March in a 2-1 over the Montreal Impact. Martínez scored his first goal of the season on 13 April, curling in a left footed shot to the side netting to give Houston a 2-1 win over San Jose. He would find the back of the net again on 27 April, this time hitting a half volley with his left foot into the left corner in a 2-0 Houston win against the Columbus Crew. On 11 June, Martínez picked up a goal an assist to help Houston get a 3-2 win over Austin Bold in a US Open Cup match. On June 18, he would find the back of the net in their next Open Cup match, however the Dynamo would lose 3-2 to Minnesota United. Martínez enjoyed his most productive month of the season in July, scoring goals in Dynamo wins against New York Red Bulls and Toronto FC as well as picking up an assist in a 3-1 defeat to LAFC. He would in the season with 7 goals and 4 assists in 37 appearances across all competitions as the Dynamo finished 10th in the Western Conference, missing out on the playoffs for the second straight season.
Personal
Martínez received his U.S. green card in January 2019, which qualifies him as a domestic player for MLS roster purposes.
