Mononen, who won the European under-23 title in Bergen a year ago, made light work of round one of the senior version, putting in a composed performance where she was constantly on the shoulders of the leaders, and was soon among a quintet of runners pulling clear of the pack.
With the top eight going through, Mononen's place in the final never looked in doubt, and a final burst of pace on the inside meant she finished second, behind Great Britain's Elise Thorner, in a time of 9:32:55.
She will now prepare for the final on 13th August, but her teammate Alisa Virtanen will not join her, as she finished 15th in the other round one race.
Kangas knocked out of medal picture
Emilia Kangas was leading a three-strong Finnish trio in the women's shot put qualification, and a best throw of 18.04m saw her finish eighth, well inside the top 12 and progressing into the evening's final. A second attempt of 18.20m put her into eighth in the final, however a foul on her third effort and Nina Chioma Ndubuisi of Germany throwing 1cm further knocked Kangas to ninth, meaning she wouldn't throw another three times.
Senja Makitorma missed out on qualifying for the final by one place, finishing 13th with a best throw of 17.38m, while Minttu Laurila finished 25th, unable to improve on her first attempt of 15.40m.
A semi-finalist at the World Indoor Championships over 60m earlier this year, Lotta Kempinen made a strong start to the women's 100m by finishing second in her heat in a season's best time of 11.25. However, in the evening session, she wasn't quite able to hit the same heights in a tough semi-final featuring home favourites Amy Hunt and Dina Asher-Smith, recording a time of 11.29 to finish seventh.
Second place in her heat behind another home favourite Lina Nielsen was more than enough for Heidi Salminen to reach the semi-finals of the women's 400m hurdles in a time of 55.93. She will be back in action on 11th August looking to reach a first senior final.
Taking to the track for the men's 5000m was former European Orienteering Championship bronze medallist Tuomas Heikkila, who set much of the pace during the first half of the race, but eventually finished 13th in a time of 13:30.64, just over 15 seconds behind the winner, Jakob Ingebrigtsen.
No joy in jumps and hammer
Finland had three representatives in the men's long jump, but none made it into the top 12 to reach the final. Kalle Salminen's second jump of 7.85m saw him finish 17th, while Kristian Pulli produced a 7.50m jump with his last effort after two fouls, and Kasperi Vehmann failed to register a mark.
Henri Liipola didn't reach the final of the men's hammer throw, as his best attempt of 71.23m was 99cm behind the final qualifer. He finished in 23rd place. Senni Salminen was below her best in the women's triple jump - 13.53m was only enough for 21st in the qualifying round.