Martens began her professional career playing for Dutch, German and Belgian clubs relatively close to her hometown of Bergen, Limburg.

Weeks after her signing at FC Barcelona was made official, Martens participated in the Netherlands' 2017 UEFA Women's Euro campaign.

She was the Netherlands' standout player at that year's Euro, where the Dutch won their first ever major international title with the women's national team.

[2] In the 2018–19 season, Martens helped FC Barcelona to their first ever UEFA Women's Champions League final, where she assisted their only goal in a 4–1 defeat to Olympique Lyonnais.

In 2021, Martens was an essential part of FC Barcelona Femení completing their first continental treble, winning the UEFA Women's Champions League, the Primera Division, and the Copa de la Reina all in the same season.

At thirteen, she went to Boxmeer club Olympia '18 [nl] at the insistence of then national-team coach Vera Pauw, where she played with teams of boys.

[7] Martens continued playing football in the Netherlands when she joined the newly-formed VVV-Venlo women's team, based in her hometown province of Limburg.

In her first official match with Standard she scored two goals to help win her first major title, the newly founded BeNe Super Cup.

Her first Champions League goal came in the second leg, where she converted a penalty to bring Standard 4–3 up, but they lost the tie on a 5–4 aggregate score.

Coach Stefan Rehn praised Martens' technique and her understanding with compatriot centre forward Manon Melis.

[19][20] In the 2016–17 UEFA Women's Champions League, she was knocked out of the competition by her future club FC Barcelona in the quarterfinals on a 3–0 aggregate score.

[33] In the 2017–18 UEFA Women's Champions League campaign, Martens recorded three assists in a win against Round of 16 opponent FC Gintra.

[37] Barcelona won 2–0, but the win wasn't enough to secure a league title, and they finished second place for the fourth season in a row.

[39] Martens helped FC Barcelona reach the final of the UEFA Women's Champions League in her second season at the club.

[51] Similarly to the 2019–20 UEFA Women's Champions League, the 2019–20 Copa de la Reina was pushed back a year, and was completed in the middle of the 2020–21 season.

[60] She assisted Barcelona's fourth and final goal, a dribble down the wing and a cross into the box that was finished by Caroline Graham Hansen.

[61] In the 2021 Copa de la Reina, Martens played in the quarterfinals of the competition against Sevilla, where she was subbed on in the 60th minute and assisted Caroline Graham Hansen's goal that put Barcelona 4–1 up.

[62] In the semifinals, Martens gave another assist, this time to Alexia Putellas who scored Barcelona's fourth goal against Madrid CFF.

Following the completion of the treble, Martens ended her season and took an early leave from the club to join her national team in preparation for the 2021 Olympics.

[68] In the same month, she was nominated as a finalist for the UEFA Women's Player of the Year Award for the second time in her career, alongside Barcelona teammates Alexia Putellas and Jenni Hermoso.

She scored within 15 seconds with a header off a corner to put the Dutch 2–0 up, and also conceded a foul that gave a French player a red card.

[79] In June 2013, national team coach Roger Reijners selected Martens in the Netherlands squad for UEFA Women's Euro 2013 in Sweden.

[86] The Dutch beat the Italians on an aggregate score of 3–2, with Martens assisting Vivianne Miedema's tie-winning goal in the second leg, sending them to compete in their first ever Women's World Cup tournament.

Martens started the final and scored the third goal of the match, a shot from the outside of the box into the lower-right corner, to bring the Netherlands 2–1 up in the 30th minute.

[101] Martens did not feature in the semi-final tie of the playoffs due to a foot injury, and the Dutch won 4–1 on aggregate against Denmark.

Martens was selected in the final squad for the 2019 FIFA Women's World Cup in France,[104] despite the fact that her large toe was injured.

[106][107] When celebrating her last-minute winner against Japan, teammate Jill Roord stepped on her foot and aggrivated her toe injury.

[109] Thanks to a goal in extra time from Jackie Groenen, the Dutch made it to their first ever Women's World Cup final against the United States.

[110][111] Martens started the final regardless, but had a hard head-to-head collision with United States defender Kelley O'Hara that caused them to both be down on the pitch for a few minutes.

[116] In the Netherlands' third and final group stage match, Martens scored another two goals against China as the Dutch advanced to the quarterfinals of the tournament.