The 23-year-old Swede took several fine scalps on his way to the semi-finals.
Among other things, he knocked out former world number one Andy Murray, now ranked 50th, the Russian 36th-ranked Aslan Karatsev and the 46th-ranked Finn Ruusuvuori.
In the semifinals, however, the Australian Nick Kyrgios was too difficult and won in two straight sets: 7-6, 6-3.
The red-hot Kyrgios then went on to play the entire tournament after winning the final against Japan's Yoshihito Nishioka.
The numbers against the 96-ranked Nishioka were 6-4, 6-3.
As with Mikael Ymer, the success in Washington meant that Nishioka also took a big step up the rankings, from 96th to 54th.
And even the winner Kyrgios, who did not lose a serve game throughout the tournament on his way to the final victory, advanced in the ranking.
The Australian picked up 26 places and is now ranked 37th in the world.
The ATP rankings are still topped by Daniil Medvedev, Russia, ahead of Germany's Alexander Zverev and Spain's Rafel Nadal.
Ymer has at best been ranked 67 in the world, at the beginning of 2020.