The milestone was celebrated at Majestic Square on Monday with young performers taking the stage and about 50 community supporters gathered.
Social worker Terry Sarten said it was a milestone significant not only to Whanganui but to all of New Zealand.
He was one of the founders of the service who spotted a gap in the system for youth aged 10-25 and became determined to make a change.
"Twenty-five years ago young people across the whole country had to get help from the hospital and that's a whole different concept ... imagine sitting in the hospital's waiting room outside the mental ward stressed because of exams or unsure what is even wrong."
Mr Sarten said the one-stop-shop was an idea created so young people could have somewhere to go that was neutral - "a place for social and health services".
When the service, previously known as Youth Advice Centre (YAC), launched in 1992 it was the first one in New Zealand.
It was initially staffed by a roster of health professionals who came into the community to provide social services for young people.
Now on Drews Ave, YST is made up of a team of nurses, GPs, counsellors, social workers and youth workers.
The service is for people aged 10-25 who enter as self-referrals, walk-ins or referrals from GPs, schools, government agencies and other NGOs.
Ms Benge, who has worked with YST for the past 12 years, said for young people especially it can be difficult to navigate their way round different services and that was why YST - the one-stop-shop - works.
"The 17-year-old woman was immediately referred to SUPP to help with her drinking, she saw a counsellor regularly and we helped get her connected into the community by getting her enrolled in a course at UCOL."
Ms Benge said giving these young people time was hugely important.
"We have extra-long appointments and if we feel the patients need more time we take what we need.
"What we do is working and it's great to be able to celebrate that."