[4] Mikael contested his first junior major final at the 2015 Wimbledon Championships but was defeated by American Reilly Opelka in straight sets.

[5] In 2015 Ymer managed to claim his second European Championships title (U18), beating Bernabé Zapata Miralles in the final in straight sets.

[3] Ymer reached the third round of a Grand Slam tournament for the first time in his career at the 2021 Australian Open.

In February, at the 2022 Open Sud de France, Ymer reached the semi-finals, defeating three French players Corentin Moutet, third seed Gaël Monfils (his second top-20 win) and Richard Gasquet.

At the 2022 Firenze Open he reached the semi-finals as a qualifier defeating again fifth seed Aslan Karatsev[17] and Roberto Carballés Baena but lost to JJ Wolf.

At his home tournament in Stockholm using a special exempt status, he reached the quarterfinals where he lost to top seed Stefanos Tsitsipas.

He continued his good form qualifying for the main draw of the 2022 Rolex Paris Masters for the second year in a row.

[21] Getting past qualifying at the second tournament, he reached the quarterfinals where he was defeated by eventual champion, Kwon Soon-woo.

[23] Making it past qualifying at the first edition of the BW Open, Ymer made it to the final where he lost to top seed David Goffin.

Representing Sweden in the Davis Cup tie against Bosnia and Herzegovina, he won both of his matches beating Mirza Bašić and Damir Džumhur.

[25][26] Sweden ended up winning the tie over Bosnia and Herzegovina 3–1 to advance to the Davis Cup Finals.

[27] At the Open Sud de France, he lost in the first round to seventh seed Emil Ruusuvuori.

Ymer was disqualified from his match at the 2023 ATP Lyon Open against Arthur Fils after smashing a racquet against the umpire's chair.

On 17 July 2023, CAS announced that Ymer will be suspended for 18 months after having missed three out-of-competition doping test attempts in a 12-month period.

The CAS reported that the younger brother was at a junior tennis tournament in Finland at the time so Mikael's version of events could not be true.