Montero played for a number of years in Portugal at Sporting CP; across two stints from 2013 to 2016 and 2018 to 2019, he netted 43 goals across all competitions for Os Leões.

Montero was loaned to Atlético Huila in 2006, where he was named top goalscorer of the 2007 Apertura, tied on 13 goals with Sergio Galván Rey.

Despite rumors and interest from European clubs,[citation needed] he returned to Deportivo Cali in 2007; he was again the top goalscorer, with 16 goals in the 2008 Finalizacion.

He made his MLS debut on 19 March 2009, in the 2009 season opener against New York Red Bulls, and scored the first and third goals in club history.

[7] In April 2009, Montero told Sports Illustrated that he would like to play in Europe in the future and that the mix of players in MLS appealed to him as a place to learn.

[9] Montero was selected to the 2009 MLS All-Star Game against Everton and played during the first half alongside teammates Kasey Keller and Jhon Kennedy Hurtado.

[11][12] In August 2009, it was incorrectly reported that MLS had negotiated the transfer of Deportivo Cali's portion of Montero's rights to Major League Soccer.

[21] On 1 December 2010, Seattle Sounders FC announced that Montero had agreed to a contract extension with his rights permanently transferred to the club and Major League Soccer.

[23] Montero was officially loaned to Colombian champions Millonarios, playing in the Categoría Primera A, on 21 January 2013,[24][25] also signing a multi-year extension to his contract with the Sounders.

[27][28] On his debut for the Portuguese club, Montero scored a hat-trick against newly promoted side Arouca, in a 5–1 win in the first round of the Primeira Liga season.

[31] On 30 January 2014, it was announced that Montero had signed a four-and-a-half-year deal with Sporting for an additional $1.55 million transfer fee plus bonuses.

[43] He had expressed a desire to return to the Seattle area and retire as a Sounder, while the club were in need of a new forward to replace the injured Jordan Morris.

Montero retook the all-time scoring record for the Sounders in April 2021, surpassing Clint Dempsey, and ended the season with two goals.

[47] After his contract with Seattle Sounders expired at the end of the 2023 season, Montero's return to Deportivo Cali was announced on 2 April 2024.

[4][50] The forward scored his first goal for Colombia in the 2nd minute of extra time against Catalonia on 29 December 2008 during a 2–1 loss at Camp Nou in Barcelona.

[44][56] Montero was named MLS Humanitarian of the Month for March 2011 after raising over $29,000 to donate towards relief for victims of the 2010 Colombian floods.

[57][58] Montero threw out first pitch for the Seattle Mariners baseball team against the Cleveland Indians on 21 August 2012, shortly after honoring Félix Hernández's perfect game in his goal celebration against the Vancouver Whitecaps FC.